diff --git a/crud.py b/crud.py
index 5a70fec..94c42e2 100644
--- a/crud.py
+++ b/crud.py
@@ -408,7 +408,16 @@ async def delete_dca_client(client_id: str) -> None:
 # =============================================================================
 
 
-async def create_deposit(creator_user_id: str, data: CreateDepositData) -> DcaDeposit:
+async def create_deposit(
+    creator_user_id: str, data: CreateDepositData, *, currency: str
+) -> DcaDeposit:
+    """Insert a deposit row.
+
+    `currency` is passed explicitly by the caller (the API endpoint
+    resolves it from the target machine's `fiat_code`) rather than
+    coming off the request body — the operator doesn't get to choose
+    it (`aiolabs/satmachineadmin#26`).
+    """
     deposit_id = urlsafe_short_hash()
     await db.execute(
         """
@@ -424,7 +433,7 @@ async def create_deposit(creator_user_id: str, data: CreateDepositData) -> DcaDe
             "machine_id": data.machine_id,
             "creator_user_id": creator_user_id,
             "amount": data.amount,
-            "currency": data.currency,
+            "currency": currency,
             "status": "pending",
             "notes": data.notes,
             "created_at": datetime.now(),
diff --git a/migrations.py b/migrations.py
index 0d9bc7d..508aa51 100644
--- a/migrations.py
+++ b/migrations.py
@@ -437,3 +437,36 @@ async def m004_introduce_dca_lp_table(db):
         "autoforward_enabled",
     ):
         await db.execute(f"ALTER TABLE satoshimachine.dca_clients DROP COLUMN {col}")
+
+
+async def m005_lock_deposit_currency_to_machine_fiat_code(db):
+    """Rewrite every `dca_deposits.currency` row to match its joined
+    `dca_machines.fiat_code`.
+
+    Today each machine handles exactly one currency (operator-set on
+    `dca_machines.fiat_code`); a deposit's currency is fully determined
+    by the machine it's recorded against. The deposit dialog was
+    historically a freeform text input, which let an operator typo a
+    currency code (e.g., a "15 USD" row landed against an EUR Sintra
+    during 2026-05-16 testing — that mismatch silently inflated the LP's
+    nominal balance because the balance summary is currency-blind).
+
+    `aiolabs/satmachineadmin#26` locks the input side; this migration
+    fixes any rows already on disk. Idempotent: on a fresh install with
+    no mismatches it's a no-op UPDATE.
+    """
+    await db.execute("""
+        UPDATE satoshimachine.dca_deposits AS d
+        SET currency = (
+            SELECT m.fiat_code
+            FROM satoshimachine.dca_machines m
+            WHERE m.id = d.machine_id
+        )
+        WHERE EXISTS (
+            SELECT 1
+            FROM satoshimachine.dca_machines m
+            WHERE m.id = d.machine_id
+              AND m.fiat_code IS NOT NULL
+              AND m.fiat_code != d.currency
+        )
+        """)
diff --git a/models.py b/models.py
index 431e70d..c61a515 100644
--- a/models.py
+++ b/models.py
@@ -165,10 +165,19 @@ class ClientBalanceSummary(BaseModel):
 
 
 class CreateDepositData(BaseModel):
+    """Operator records a fiat deposit against an LP enrolment.
+
+    `currency` is server-set from the target machine's `fiat_code` at
+    write time — the API ignores any value the client submits. Each
+    machine currently handles exactly one currency (`dca_machines.
+    fiat_code`); allowing the operator to pick a different one at
+    deposit time would either be a typo or a future multi-currency
+    feature that doesn't exist yet (`aiolabs/satmachineadmin#26`).
+    """
+
     client_id: str
     machine_id: str
     amount: float
-    currency: str = "GTQ"
     notes: Optional[str] = None
 
     @validator("amount")
@@ -192,8 +201,11 @@ class DcaDeposit(BaseModel):
 
 
 class UpdateDepositData(BaseModel):
+    """Operator edits on a pending deposit. `currency` removed — see
+    `CreateDepositData`; the currency is bound to the machine and not
+    editable after the row lands."""
+
     amount: Optional[float] = None
-    currency: Optional[str] = None
     notes: Optional[str] = None
 
     @validator("amount")
diff --git a/static/js/index.js b/static/js/index.js
index fb2a470..f67aa46 100644
--- a/static/js/index.js
+++ b/static/js/index.js
@@ -278,6 +278,15 @@ window.app = Vue.createApp({
       const id = this.depositDialog.data.client_id
       return id ? this.clients.find(c => c.id === id) : null
     },
+    depositMachineFiatCode() {
+      // Currency the deposit will land in — bound to the machine the
+      // selected LP is enrolled at. Resolved entirely client-side from
+      // already-loaded data, but the server has the final say (#26).
+      const c = this.selectedDepositClient
+      if (!c) return null
+      const m = this.machines.find(m => m.id === c.machine_id)
+      return m ? m.fiat_code : null
+    },
     worklistBuckets() {
       return [
         {
@@ -966,7 +975,6 @@ window.app = Vue.createApp({
         id: deposit.id,
         client_id: deposit.client_id,
         amount: deposit.amount,
-        currency: deposit.currency,
         notes: deposit.notes || ''
       }
       this.depositDialog.show = true
@@ -978,13 +986,14 @@ window.app = Vue.createApp({
       try {
         if (this.depositDialog.mode === 'add') {
           // machine_id is server-cross-checked but we send it explicitly.
+          // currency is server-resolved from the machine's fiat_code
+          // (#26); not in the request body.
           const client = this.clients.find(c => c.id === d.client_id)
           if (!client) throw new Error('client not found')
           const body = {
             client_id: d.client_id,
             machine_id: client.machine_id,
             amount: Number(d.amount),
-            currency: (d.currency || 'GTQ').trim(),
             notes: (d.notes || '').trim() || null
           }
           const {data} = await LNbits.api.request('POST', DEPOSITS_PATH, null, body)
@@ -993,7 +1002,6 @@ window.app = Vue.createApp({
         } else {
           const body = {
             amount: Number(d.amount),
-            currency: (d.currency || 'GTQ').trim(),
             notes: (d.notes || '').trim() || null
           }
           const {data} = await LNbits.api.request(
@@ -1279,10 +1287,12 @@ window.app = Vue.createApp({
     },
 
     _emptyDepositForm() {
+      // currency is server-resolved from the selected client's machine
+      // fiat_code (see #26); not stored on the form, just displayed in
+      // the dialog via depositMachineFiatCode() computed.
       return {
         client_id: null,
         amount: null,
-        currency: 'GTQ',
         notes: ''
       }
     },
diff --git a/templates/satmachineadmin/index.html b/templates/satmachineadmin/index.html
index 5972481..c43450c 100644
--- a/templates/satmachineadmin/index.html
+++ b/templates/satmachineadmin/index.html
@@ -1136,14 +1136,23 @@
           </q-banner>
 
           <q-input v-model.number="depositDialog.data.amount"
-            label="Amount (fiat)"
+            :label="depositMachineFiatCode ? `Amount (${depositMachineFiatCode})` : 'Amount (fiat)'"
             type="number" step="0.01" min="0"
             class="q-mb-md" dense outlined
-            :rules="[v => v > 0 || 'Must be > 0']"></q-input>
-
-          <q-input v-model="depositDialog.data.currency"
-            label="Currency"
-            class="q-mb-md" dense outlined></q-input>
+            :rules="[v => v > 0 || 'Must be > 0']">
+            <template v-slot:append>
+              <q-chip v-if="depositMachineFiatCode"
+                      dense color="grey-3" text-color="grey-9"
+                      :style="{fontWeight: 500}"
+                      v-text="depositMachineFiatCode">
+                <q-tooltip>
+                  Currency is set by the selected LP's machine
+                  (<code v-text="depositMachineFiatCode"></code>) — not
+                  operator-editable. See aiolabs/satmachineadmin#26.
+                </q-tooltip>
+              </q-chip>
+            </template>
+          </q-input>
 
           <q-input v-model="depositDialog.data.notes"
             label="Notes (optional)"
diff --git a/tests/test_deposit_currency.py b/tests/test_deposit_currency.py
new file mode 100644
index 0000000..fd0be00
--- /dev/null
+++ b/tests/test_deposit_currency.py
@@ -0,0 +1,56 @@
+"""
+Locks in the contract from `aiolabs/satmachineadmin#26`: a deposit's
+currency is bound to its machine's `fiat_code`, never operator-chooseable.
+
+The mechanism is two-layered:
+  1. `CreateDepositData` / `UpdateDepositData` Pydantic models don't
+     accept a `currency` field — any value a client submits is dropped
+     at validation, before reaching the handler.
+  2. The `api_create_deposit` endpoint resolves the machine's
+     `fiat_code` server-side and passes it to `create_deposit(
+     ..., currency=...)`.
+
+This test covers layer 1 (the model contract). Layer 2 is an
+endpoint-level behaviour better covered by an integration test against
+a running LNbits; tracked in #26 as a follow-up.
+"""
+
+from ..models import CreateDepositData, UpdateDepositData
+
+
+def test_create_deposit_data_has_no_currency_field():
+    """A client posting `{currency: "USD"}` against an EUR machine must
+    have that field silently dropped by validation — there's no public
+    way to inject the wrong currency through this endpoint."""
+    fields = CreateDepositData.__fields__
+    assert "currency" not in fields, (
+        f"CreateDepositData must not expose a `currency` field "
+        f"(found {list(fields)})"
+    )
+
+
+def test_update_deposit_data_has_no_currency_field():
+    """Same protection on the edit path: a pending deposit can have
+    its amount / notes edited, but never its currency — that's bound
+    to the machine."""
+    fields = UpdateDepositData.__fields__
+    assert "currency" not in fields, (
+        f"UpdateDepositData must not expose a `currency` field "
+        f"(found {list(fields)})"
+    )
+
+
+def test_create_deposit_data_drops_unknown_currency_silently():
+    """Pydantic's default `Config` ignores unknown fields, so a stray
+    `currency` on the request body parses cleanly without leaving
+    a trace on the resulting model. Belt-and-braces — locks in the
+    "input has no way to influence the currency" guarantee."""
+    data = CreateDepositData(
+        client_id="c1",
+        machine_id="m1",
+        amount=20.0,
+        currency="USD",  # ignored — field doesn't exist on the model
+    )
+    assert not hasattr(data, "currency")
+    assert data.amount == 20.0
+    assert data.machine_id == "m1"
diff --git a/views_api.py b/views_api.py
index 8b6db83..1beb7ac 100644
--- a/views_api.py
+++ b/views_api.py
@@ -324,7 +324,14 @@ async def api_create_deposit(
             "satmachineclient and select a DCA wallet before deposits "
             "can be recorded against them.",
         )
-    return await create_deposit(user.id, data)
+    # Currency is bound to the machine, not operator-choosable. Resolve
+    # it server-side so an operator with a UI bug / curl mistake / older
+    # client can't poison the LP balance with the wrong unit
+    # (aiolabs/satmachineadmin#26).
+    machine = await get_machine(data.machine_id)
+    if machine is None:
+        raise HTTPException(HTTPStatus.NOT_FOUND, "Machine not found")
+    return await create_deposit(user.id, data, currency=machine.fiat_code)
 
 
 @satmachineadmin_api_router.get("/api/v1/dca/deposits", response_model=list[DcaDeposit])