diff --git a/migrations.py b/migrations.py
index 282e1fc..b82e7d1 100644
--- a/migrations.py
+++ b/migrations.py
@@ -88,3 +88,27 @@ async def m002_payouts(db):
         "CREATE INDEX payroll.idx_payouts_employee_wallet "
         "ON payouts (employee_wallet);"
     )
+
+
+async def m003_pricing_mode(db):
+    """Let a back-dated period be priced at the day it was due.
+
+    Until now every payout converted at whatever the rate was when it
+    settled, which silently misprices a period paid late. The contract now
+    carries how to convert, and each payout records the rate it actually
+    used plus where that rate came from — so a figure in the ledger can be
+    explained months later instead of merely trusted.
+
+    Defaults preserve existing behaviour for anything already in flight:
+    every live payday is today or ahead, and for those all three modes agree
+    on using LNbits' own live pricing.
+    """
+    await db.execute(
+        "ALTER TABLE payroll.contracts "
+        "ADD COLUMN pricing_mode TEXT NOT NULL DEFAULT 'payday';"
+    )
+    await db.execute("ALTER TABLE payroll.contracts ADD COLUMN manual_rate REAL;")
+    await db.execute("ALTER TABLE payroll.payouts ADD COLUMN rate REAL;")
+    await db.execute(
+        "ALTER TABLE payroll.payouts ADD COLUMN rate_source TEXT NOT NULL DEFAULT '';"
+    )
diff --git a/models.py b/models.py
index 15ccaac..524224f 100644
--- a/models.py
+++ b/models.py
@@ -55,6 +55,25 @@ class Frequency(str, Enum):
     yearly = "yearly"
 
 
+class PricingMode(str, Enum):
+    """How a period's fiat amount becomes a sat amount.
+
+    Only ever matters for a payday in the past — for a payday that is today
+    or ahead, all three agree and LNbits' own live pricing is used.
+    """
+
+    # Convert at what BTC was worth on the payday itself. The default: a
+    # back-dated period is priced as of when it was due, not when somebody
+    # got round to paying it.
+    payday = "payday"
+    # Convert at today's rate. Correct when the obligation is understood as
+    # "we owe them EUR 800, whenever it settles".
+    current = "current"
+    # Convert at a rate the operator states outright (e.g. 100000 EUR/BTC),
+    # for a figure that was agreed rather than looked up.
+    manual = "manual"
+
+
 class ContractStatus(str, Enum):
     active = "active"  # eligible for payout on every scheduler tick
     paused = "paused"  # keeps its schedule position, pays nothing
@@ -101,6 +120,19 @@ class CreateContract(BaseModel):
     # months of back-pay in one tick.
     backfill: bool = False
 
+    # How to price a back-dated period. Irrelevant for a sat-denominated
+    # contract, and irrelevant for any payday that is not in the past.
+    pricing_mode: PricingMode = PricingMode.payday
+    # Units of `currency` per whole BTC. Required by, and only read under,
+    # PricingMode.manual.
+    manual_rate: float | None = None
+
+    @validator("manual_rate")
+    def _positive_rate(cls, v: float | None) -> float | None:
+        if v is not None and v <= 0:
+            raise ValueError("manual_rate must be positive")
+        return v
+
     @validator("start_date")
     def _valid_date(cls, v: str) -> str:
         datetime.strptime(v, "%Y-%m-%d")  # raises for anything malformed
@@ -159,6 +191,8 @@ class UpdateContract(BaseModel):
     total_periods: int | None = None
     label: str | None = None
     memo: str | None = None
+    pricing_mode: PricingMode | None = None
+    manual_rate: float | None = None
 
     @validator("amount")
     def _positive_amount(cls, v: float | None) -> float | None:
@@ -239,6 +273,14 @@ class Payout(BaseModel):
     payment_hash: str | None = None
     detail: str = ""
 
+    # Units of `currency` per whole BTC actually used for this period, and
+    # which of the pricing modes produced it. For a live conversion the rate
+    # is read back off the invoice LNbits priced rather than recomputed, so
+    # the row records what happened and not an approximation of it. Null on
+    # a sat contract, and on a failure that never got as far as pricing.
+    rate: float | None = None
+    rate_source: str = ""  # "current" | "payday" | "manual" | "" (sat / none)
+
     created_at: datetime = Field(default_factory=_now)
 
     @property
diff --git a/services.py b/services.py
index f167e8c..f1123a0 100644
--- a/services.py
+++ b/services.py
@@ -33,7 +33,9 @@ from .models import (
     Frequency,
     Payout,
     PayoutStatus,
+    PricingMode,
 )
+from .rates import historical_btc_rate, sats_for
 
 # Frequencies that are an exact number of days: no calendar involved, so no
 # clamping is possible or needed.
@@ -187,6 +189,8 @@ class PeriodOutcome:
     detail: str = ""
     amount_msat: int | None = None
     payment_hash: str | None = None
+    rate: float | None = None
+    rate_source: str = ""
 
     @property
     def consumed(self) -> bool:
@@ -214,12 +218,86 @@ def _lock_for(contract_id: str) -> asyncio.Lock:
     return lock
 
 
+def _today() -> date:
+    return datetime.now(timezone.utc).date()
+
+
 def payout_memo(contract: Contract, payday: date) -> str:
     label = contract.memo or contract.label or "Payroll"
     return f"{label} — {payday.isoformat()}"
 
 
-async def pay_period(contract: Contract, index: int, payday: date) -> PeriodOutcome:
+class PricingError(ValueError):
+    """A period's sat amount could not be established.
+
+    Deliberately fatal for that period. The alternative — falling back to
+    today's rate when the payday's rate is unavailable — pays a materially
+    different amount than intended and hides it, which is exactly the class
+    of error nobody finds until an audit.
+    """
+
+
+@dataclass
+class Price:
+    """What to hand `create_invoice`, plus the provenance to record.
+
+    When `currency` is the contract's own, LNbits does the conversion and
+    `rate` is filled in afterwards from the invoice it priced — read back
+    rather than recomputed, so the ledger records what actually happened.
+    """
+
+    amount: float
+    currency: str
+    rate: float | None
+    source: str
+
+
+async def resolve_price(
+    contract: Contract,
+    payday: date,
+    today: date,
+    mode: PricingMode | None = None,
+    manual_rate: float | None = None,
+) -> Price:
+    """Decide what a period is worth, and say where the number came from.
+
+    `mode`/`manual_rate` override the contract's own settings for a single
+    call, which is how an operator prices one off-cycle payout differently
+    without editing the contract.
+    """
+    if contract.currency.lower() == "sat":
+        return Price(contract.amount, "sat", None, "")
+
+    mode = mode or contract.pricing_mode
+    manual_rate = manual_rate or contract.manual_rate
+
+    # A payday that is today or ahead needs no history: every mode agrees,
+    # and LNbits' own live pricing is the freshest source available.
+    if payday >= today or mode == PricingMode.current:
+        return Price(contract.amount, contract.currency, None, "current")
+
+    if mode == PricingMode.manual:
+        if not manual_rate:
+            raise PricingError("manual pricing selected but no rate given")
+        return Price(
+            sats_for(contract.amount, manual_rate), "sat", manual_rate, "manual"
+        )
+
+    rate = await historical_btc_rate(payday, contract.currency)
+    if rate is None:
+        raise PricingError(
+            f"no historical {contract.currency} rate available for {payday}"
+        )
+    return Price(sats_for(contract.amount, rate), "sat", rate, "payday")
+
+
+async def pay_period(
+    contract: Contract,
+    index: int,
+    payday: date,
+    mode: PricingMode | None = None,
+    manual_rate: float | None = None,
+) -> PeriodOutcome:
     """Move one period's money from the source wallet to the employee wallet.
 
     An internal invoice on the destination wallet, paid from the source
@@ -240,11 +318,23 @@ async def pay_period(contract: Contract, index: int, payday: date) -> PeriodOutc
         "payday": payday.isoformat(),
     }
 
+    try:
+        price = await resolve_price(contract, payday, _today(), mode, manual_rate)
+    except (PricingError, ValueError) as exc:
+        return PeriodOutcome(
+            index=index,
+            payday=payday,
+            status=PayoutStatus.failed,
+            detail=str(exc),
+        )
+    if price.source:
+        extra["rate_source"] = price.source
+
     try:
         invoice = await create_invoice(
             wallet_id=contract.employee_wallet,
-            amount=contract.amount,
-            currency=contract.currency,
+            amount=price.amount,
+            currency=price.currency,
             memo=memo,
             internal=True,
             extra=extra,
@@ -262,6 +352,13 @@ async def pay_period(contract: Contract, index: int, payday: date) -> PeriodOutc
     # invoice.amount is msat and is now the canonical figure for this period.
     amount_msat = invoice.amount
 
+    # For a live conversion LNbits stashes the rate it used on the payment;
+    # read it back rather than recomputing, so the ledger records the number
+    # that was actually applied.
+    rate = price.rate
+    if rate is None and price.source == "current":
+        rate = (invoice.extra or {}).get("btc_rate")
+
     source = await get_wallet(contract.source_wallet)
     if not source:
         return PeriodOutcome(
@@ -270,6 +367,8 @@ async def pay_period(contract: Contract, index: int, payday: date) -> PeriodOutc
             status=PayoutStatus.failed,
             detail="source wallet not found",
             amount_msat=amount_msat,
+            rate=rate,
+            rate_source=price.source,
         )
     if source.balance_msat < amount_msat:
         return PeriodOutcome(
@@ -282,6 +381,8 @@ async def pay_period(contract: Contract, index: int, payday: date) -> PeriodOutc
                 f"{amount_msat // 1000} sat required"
             ),
             amount_msat=amount_msat,
+            rate=rate,
+            rate_source=price.source,
         )
 
     try:
@@ -300,6 +401,8 @@ async def pay_period(contract: Contract, index: int, payday: date) -> PeriodOutc
             detail=f"payment failed: {exc}",
             amount_msat=amount_msat,
             payment_hash=invoice.payment_hash,
+            rate=rate,
+            rate_source=price.source,
         )
 
     return PeriodOutcome(
@@ -308,6 +411,8 @@ async def pay_period(contract: Contract, index: int, payday: date) -> PeriodOutc
         status=PayoutStatus.paid,
         amount_msat=amount_msat,
         payment_hash=invoice.payment_hash,
+        rate=rate,
+        rate_source=price.source,
     )
 
 
@@ -324,7 +429,7 @@ async def run_due_periods(
     retried on the next tick, because a failed period does not advance the
     contract's position.
     """
-    today = today or datetime.now(timezone.utc).date()
+    today = today or _today()
 
     async with _lock_for(contract.id):
         # Re-read under the lock: a manual run may have moved the position
@@ -386,6 +491,8 @@ async def _record(contract: Contract, outcome: PeriodOutcome) -> Payout:
             source_wallet=contract.source_wallet,
             payment_hash=outcome.payment_hash,
             detail=outcome.detail,
+            rate=outcome.rate,
+            rate_source=outcome.rate_source,
         )
     )
 
@@ -522,7 +629,7 @@ def resume(
 
     contract.status = ContractStatus.active
     if not catch_up:
-        today = today or datetime.now(timezone.utc).date()
+        today = today or _today()
         skipped_to = fast_forward_index(contract, today)
         if skipped_to != contract.periods_done:
             logger.info(
@@ -551,7 +658,11 @@ def cancel(contract: Contract) -> Contract:
 # ---------------------------------------------------------------------------
 
 
-async def pay_now(contract: Contract) -> Payout:
+async def pay_now(
+    contract: Contract,
+    mode: PricingMode | None = None,
+    manual_rate: float | None = None,
+) -> Payout:
     """Settle the contract's next period immediately, whatever the calendar
     says.
 
@@ -565,6 +676,10 @@ async def pay_now(contract: Contract) -> Payout:
     because `_settle`'s back-dated skip exists to stop a *new* contract
     firing surprise back-pay. An operator explicitly asking to pay a period
     is not a surprise.
+
+    `mode`/`manual_rate` price this one payout differently without editing
+    the contract — the case an operator hits when entering a payment that
+    happened weeks ago at a rate they already know.
     """
     async with _lock_for(contract.id):
         fresh = await crud.get_contract(contract.id)
@@ -582,9 +697,9 @@ async def pay_now(contract: Contract) -> Payout:
 
         index = contract.periods_done
         payday = occurrence_on(parse_start_date(contract), contract.frequency, index)
-        today = datetime.now(timezone.utc).date()
+        today = _today()
 
-        outcome = await pay_period(contract, index, payday)
+        outcome = await pay_period(contract, index, payday, mode, manual_rate)
         if payday > today and outcome.status == PayoutStatus.paid:
             outcome.detail = f"paid early on {today.isoformat()} (due {payday})"
 
diff --git a/tests/conftest.py b/tests/conftest.py
index 3bd7a28..99fc158 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -7,7 +7,7 @@ keeping `services` split into a pure half and an effectful half.
 
 from datetime import datetime, timezone
 
-from ..models import Contract, ContractStatus, Frequency
+from ..models import Contract, ContractStatus, Frequency, PricingMode
 
 
 def make_contract(
@@ -22,6 +22,8 @@ def make_contract(
     currency: str = "sat",
     backfill: bool = False,
     created_at: str = "2026-01-01",
+    pricing_mode: PricingMode = PricingMode.payday,
+    manual_rate: float | None = None,
 ) -> Contract:
     return Contract(
         id=contract_id,
@@ -37,6 +39,8 @@ def make_contract(
         periods_done=periods_done,
         status=status,
         backfill=backfill,
+        pricing_mode=pricing_mode,
+        manual_rate=manual_rate,
         created_at=datetime.strptime(created_at, "%Y-%m-%d").replace(
             tzinfo=timezone.utc
         ),
diff --git a/tests/test_payout.py b/tests/test_payout.py
index defff71..0157481 100644
--- a/tests/test_payout.py
+++ b/tests/test_payout.py
@@ -57,7 +57,7 @@ def payroll_stub(monkeypatch):
     async def fake_count_period_failures(_contract_id, _period_index):
         return stub.prior_failures
 
-    async def fake_pay_period(contract, index, payday):
+    async def fake_pay_period(contract, index, payday, mode=None, manual_rate=None):
         stub.attempted.append(index)
         paid = stub.outcome_status == PayoutStatus.paid
         return PeriodOutcome(
diff --git a/tests/test_pricing.py b/tests/test_pricing.py
new file mode 100644
index 0000000..8c5c908
--- /dev/null
+++ b/tests/test_pricing.py
@@ -0,0 +1,131 @@
+"""Pricing a period.
+
+`resolve_price` is where a back-dated payout stops being worth today's
+money. These pin down which mode wins when, and that an unavailable rate
+refuses rather than substitutes.
+"""
+
+import asyncio
+from datetime import date
+
+import pytest
+
+from .. import services
+from ..models import PricingMode
+from ..services import PricingError, resolve_price
+from .conftest import make_contract
+
+TODAY = date(2026, 8, 31)
+PAST = date(2026, 5, 15)
+PAST_RATE = 69743.26
+
+
+@pytest.fixture
+def historical(monkeypatch):
+    """Stub the lookup and record what it was asked for."""
+    asked = []
+
+    async def fake_rate(day, currency):
+        asked.append((day, currency))
+        return PAST_RATE if currency == "EUR" else None
+
+    monkeypatch.setattr(services, "historical_btc_rate", fake_rate)
+    return asked
+
+
+def eur(**kw):
+    return make_contract(currency="EUR", amount=800, **kw)
+
+
+def price(contract, payday=PAST, today=TODAY, **kw):
+    return asyncio.run(resolve_price(contract, payday, today, **kw))
+
+
+# --- when history is irrelevant --------------------------------------------
+
+
+def test_a_payday_today_uses_live_pricing(historical):
+    """No mode needs history for a payday that has not passed — LNbits' own
+    conversion is the freshest source there is."""
+    p = price(eur(), payday=TODAY)
+    assert (p.currency, p.source) == ("EUR", "current")
+    assert p.amount == 800  # handed to create_invoice as fiat, unconverted
+    assert historical == []
+
+
+def test_a_future_payday_uses_live_pricing(historical):
+    p = price(eur(), payday=date(2026, 12, 1))
+    assert p.source == "current"
+    assert historical == []
+
+
+def test_a_sat_contract_never_prices_at_all(historical):
+    p = price(make_contract(currency="sat", amount=1000))
+    assert (p.amount, p.currency, p.rate, p.source) == (1000, "sat", None, "")
+    assert historical == []
+
+
+# --- the three modes -------------------------------------------------------
+
+
+def test_payday_mode_converts_at_the_paydays_rate(historical):
+    p = price(eur(pricing_mode=PricingMode.payday))
+    assert p.source == "payday"
+    assert p.currency == "sat"
+    assert p.rate == PAST_RATE
+    assert p.amount == round(800 / PAST_RATE * 1e8)
+    assert historical == [(PAST, "EUR")]
+
+
+def test_current_mode_ignores_the_past_payday(historical):
+    """The employee is owed EUR 800 and gets EUR 800 of bitcoin today."""
+    p = price(eur(pricing_mode=PricingMode.current))
+    assert (p.currency, p.source) == ("EUR", "current")
+    assert historical == []
+
+
+def test_manual_mode_uses_the_stated_rate(historical):
+    p = price(eur(pricing_mode=PricingMode.manual, manual_rate=100_000))
+    assert p.source == "manual"
+    assert p.rate == 100_000
+    assert p.amount == 800_000  # 800 EUR at 100k EUR/BTC == 0.008 BTC
+    assert historical == []
+
+
+def test_manual_mode_without_a_rate_refuses(historical):
+    with pytest.raises(PricingError):
+        price(eur(pricing_mode=PricingMode.manual))
+
+
+# --- per-call override -----------------------------------------------------
+
+
+def test_an_override_beats_the_contract(historical):
+    """Pricing one off-cycle payout differently must not require editing the
+    contract."""
+    contract = eur(pricing_mode=PricingMode.payday)
+    p = price(contract, mode=PricingMode.manual, manual_rate=50_000)
+    assert (p.source, p.rate) == ("manual", 50_000)
+    assert historical == []
+
+
+def test_an_override_rate_supplies_a_contract_that_has_none(historical):
+    p = price(eur(), mode=PricingMode.manual, manual_rate=50_000)
+    assert p.amount == round(800 / 50_000 * 1e8)
+
+
+# --- the refusal that matters ----------------------------------------------
+
+
+def test_an_unavailable_rate_refuses_rather_than_substituting(historical):
+    """The whole point: no silent fallback to today's rate. A rate that
+    moved 30% since the payday would otherwise pay 30% off, discovered only
+    on review."""
+    with pytest.raises(PricingError):
+        price(make_contract(currency="JPY", amount=800))
+
+
+def test_the_refusal_names_the_date_and_currency(historical):
+    with pytest.raises(PricingError) as exc:
+        price(make_contract(currency="JPY", amount=800))
+    assert "JPY" in str(exc.value) and "2026-05-15" in str(exc.value)
diff --git a/views_api.py b/views_api.py
index 7218974..2d82d6d 100644
--- a/views_api.py
+++ b/views_api.py
@@ -30,6 +30,7 @@ from .models import (
     DirectoryUser,
     Payout,
     PayoutStatus,
+    PricingMode,
     SchedulePreview,
     SchedulePreviewRequest,
     UpdateContract,
@@ -95,6 +96,14 @@ async def _validate_terms(data: CreateContract) -> str:
             HTTPStatus.BAD_REQUEST, f"Unsupported currency '{data.currency}'."
         )
 
+    # Catch this now rather than as a failed payout weeks later, when the
+    # first back-dated period tries to price itself and finds no rate.
+    if data.pricing_mode == PricingMode.manual and not data.manual_rate:
+        raise HTTPException(
+            HTTPStatus.BAD_REQUEST,
+            "Manual pricing needs a rate (units of the currency per BTC).",
+        )
+
     return employee.display_name
 
 
@@ -301,7 +310,11 @@ async def api_contract_schedule(contract_id: str, count: int = 12) -> SchedulePr
 
 
 @payroll_api_router.post("/api/v1/contracts/{contract_id}/pay-now")
-async def api_pay_now(contract_id: str) -> Payout:
+async def api_pay_now(
+    contract_id: str,
+    pricing_mode: PricingMode | None = None,
+    manual_rate: float | None = None,
+) -> Payout:
     """Settle the next period immediately, whatever the calendar says.
 
     Covers both "run it now" (do not wait for the tick) and "pay it early",
@@ -312,12 +325,16 @@ async def api_pay_now(contract_id: str) -> Payout:
     Recorded in the ledger like any other payout, with the early-payment
     noted in its detail. Returns the ledger row, including a failed one:
     the caller wants to know *why* a manual payout did not land.
+
+    `pricing_mode`/`manual_rate` price this one payout differently without
+    editing the contract — for entering a payment that happened weeks ago at
+    a rate the operator already knows. They apply to this call only.
     """
     contract = await crud.get_contract(contract_id)
     if not contract:
         raise HTTPException(HTTPStatus.NOT_FOUND, "Contract not found.")
     try:
-        return await services.pay_now(contract)
+        return await services.pay_now(contract, pricing_mode, manual_rate)
     except services.LifecycleError as exc:
         raise HTTPException(HTTPStatus.CONFLICT, str(exc)) from exc