diff --git a/apps/machine/electron/__tests__/state-store-transactions.test.ts b/apps/machine/electron/__tests__/state-store-transactions.test.ts
new file mode 100644
index 0000000..fbb9a87
--- /dev/null
+++ b/apps/machine/electron/__tests__/state-store-transactions.test.ts
@@ -0,0 +1,170 @@
+/**
+ * Tests for recordTransaction inventory accounting.
+ *
+ * Regression coverage for the position-vs-denomination decrement bug:
+ * position is the cassettes PK (v9) and duplicate denominations across
+ * bays are legal, so cash-out decrements MUST address bays by position.
+ * A denomination-keyed UPDATE would drain every matching bay at once.
+ *
+ * Uses an in-memory SQLite database — fresh per test, no on-disk
+ * artifacts, no parallel-test interference.
+ */
+
+import { afterEach, beforeEach, describe, expect, it } from 'vitest'
+import {
+  closeDatabase,
+  getCashbox,
+  initDatabase,
+  loadCassettes,
+  recordTransaction,
+  setCassettes,
+} from '../state-store.js'
+
+const TX_BASE = {
+  fiatCents: 4000,
+  sats: 100_000,
+  feeSats: 5_000,
+  feeFraction: 0.05,
+  exchangeRate: 2500,
+  currency: 'USD',
+}
+
+/** Two $20 bays plus one $50 bay — the duplicate-denomination layout. */
+function seedDuplicateDenomBays() {
+  setCassettes([
+    { position: 1, denomination: 20, count: 50 },
+    { position: 2, denomination: 20, count: 50 },
+    { position: 3, denomination: 50, count: 30 },
+  ])
+}
+
+function countsByPosition(): Record<number, number> {
+  const out: Record<number, number> = {}
+  for (const row of loadCassettes()) out[row.position] = row.count
+  return out
+}
+
+beforeEach(() => {
+  initDatabase(':memory:')
+  seedDuplicateDenomBays()
+})
+afterEach(() => {
+  closeDatabase()
+})
+
+describe('state-store: recordTransaction cash_out inventory', () => {
+  it('decrements only the bay that actually dispensed (duplicate denominations)', () => {
+    recordTransaction({
+      ...TX_BASE,
+      txid: 'tx-single-bay',
+      type: 'cash_out',
+      status: 'complete',
+      bills: [{ denomination: 20, count: 3 }],
+      cassettes: [
+        {
+          name: 'cassette1',
+          position: 1,
+          denomination: 20,
+          provisioned: 3,
+          dispensed: 3,
+          rejected: 0,
+        },
+        {
+          name: 'cassette2',
+          position: 2,
+          denomination: 20,
+          provisioned: 0,
+          dispensed: 0,
+          rejected: 0,
+        },
+      ],
+    })
+
+    expect(countsByPosition()).toEqual({ 1: 47, 2: 50, 3: 30 })
+  })
+
+  it('decrements each bay by its own dispensed count on a split dispense', () => {
+    recordTransaction({
+      ...TX_BASE,
+      txid: 'tx-split-bays',
+      type: 'cash_out',
+      status: 'complete',
+      bills: [{ denomination: 20, count: 60 }],
+      cassettes: [
+        {
+          name: 'cassette1',
+          position: 1,
+          denomination: 20,
+          provisioned: 50,
+          dispensed: 50,
+          rejected: 0,
+        },
+        {
+          name: 'cassette2',
+          position: 2,
+          denomination: 20,
+          provisioned: 10,
+          dispensed: 10,
+          rejected: 0,
+        },
+      ],
+    })
+
+    expect(countsByPosition()).toEqual({ 1: 0, 2: 40, 3: 30 })
+  })
+
+  it('fallback without cassette results drains matching bays greedily by position', () => {
+    recordTransaction({
+      ...TX_BASE,
+      txid: 'tx-fallback',
+      type: 'cash_out',
+      status: 'complete',
+      bills: [{ denomination: 20, count: 60 }],
+    })
+
+    // Bay 1 (50 bills) drains fully, bay 2 covers the remaining 10.
+    expect(countsByPosition()).toEqual({ 1: 0, 2: 40, 3: 30 })
+  })
+
+  it('never drives a bay count below zero', () => {
+    recordTransaction({
+      ...TX_BASE,
+      txid: 'tx-overdispense',
+      type: 'cash_out',
+      status: 'complete',
+      bills: [{ denomination: 50, count: 35 }],
+      cassettes: [
+        {
+          name: 'cassette3',
+          position: 3,
+          denomination: 50,
+          provisioned: 35,
+          dispensed: 35,
+          rejected: 0,
+        },
+      ],
+    })
+
+    expect(countsByPosition()).toEqual({ 1: 50, 2: 50, 3: 0 })
+  })
+})
+
+describe('state-store: recordTransaction cash_in cashbox', () => {
+  it('adds inserted bills to the cashbox and leaves cassettes untouched', () => {
+    recordTransaction({
+      ...TX_BASE,
+      txid: 'tx-cash-in',
+      type: 'cash_in',
+      status: 'complete',
+      bills: [
+        { denomination: 20, count: 2 },
+        { denomination: 50, count: 1 },
+      ],
+    })
+
+    const cashbox = getCashbox()
+    expect(cashbox.totalBills).toBe(3)
+    expect(cashbox.totalFiatCents).toBe(TX_BASE.fiatCents)
+    expect(countsByPosition()).toEqual({ 1: 50, 2: 50, 3: 30 })
+  })
+})
diff --git a/apps/machine/electron/state-store.ts b/apps/machine/electron/state-store.ts
index a274565..cfbfd71 100644
--- a/apps/machine/electron/state-store.ts
+++ b/apps/machine/electron/state-store.ts
@@ -982,8 +982,11 @@ export function recordTransaction(tx: TransactionInput): void {
   const insertCassetteBill = db.prepare(
     'INSERT INTO cassette_bills (txid, name, position, denomination, provisioned, dispensed, rejected) VALUES (?, ?, ?, ?, ?, ?, ?)'
   )
-  const updateCassette = db.prepare(
-    'UPDATE cassettes SET count = MAX(0, count + ?) WHERE denomination = ?'
+  const updateCassetteByPosition = db.prepare(
+    'UPDATE cassettes SET count = MAX(0, count + ?) WHERE position = ?'
+  )
+  const selectBaysByDenom = db.prepare(
+    'SELECT position, count FROM cassettes WHERE denomination = ? ORDER BY position'
   )
   const updateCashboxStmt = db.prepare(
     'UPDATE cashbox SET total_bills = total_bills + ?, total_fiat_cents = total_fiat_cents + ? WHERE id = 1'
@@ -1024,17 +1027,33 @@ export function recordTransaction(tx: TransactionInput): void {
     }
 
     if (t.type === 'cash_out') {
-      // Decrement cassettes by ACTUALLY dispensed count (not requested)
+      // Decrement cassettes by ACTUALLY dispensed count (not requested).
+      // Position is the addressable unit (v9): duplicate denominations
+      // across bays are legal, so a denomination-keyed UPDATE would
+      // decrement every matching bay.
       if (t.cassettes) {
         for (const c of t.cassettes) {
           if (c.dispensed > 0) {
-            updateCassette.run(-c.dispensed, c.denomination)
+            updateCassetteByPosition.run(-c.dispensed, c.position)
           }
         }
       } else {
-        // Fallback: use bill counts (backward compat for mocks without cassette data)
+        // Fallback: per-denomination bill counts (mocks without per-bay
+        // results). Drain matching bays greedily in position order —
+        // the dispenser's own fill order.
         for (const bill of t.bills) {
-          updateCassette.run(-bill.count, bill.denomination)
+          let remaining = bill.count
+          const bays = selectBaysByDenom.all(bill.denomination) as {
+            position: number
+            count: number
+          }[]
+          for (const bay of bays) {
+            if (remaining <= 0) break
+            const take = Math.min(remaining, bay.count)
+            if (take <= 0) continue
+            updateCassetteByPosition.run(-take, bay.position)
+            remaining -= take
+          }
         }
       }
     }