diff --git a/apps/machine/src/views/CashInView.vue b/apps/machine/src/views/CashInView.vue
index 48c6d02..d907f15 100644
--- a/apps/machine/src/views/CashInView.vue
+++ b/apps/machine/src/views/CashInView.vue
@@ -114,6 +114,23 @@ function skipReceipt() {
   atmStore.skipReceipt()
 }
 
+// Check if a bill denomination can still be accepted without exceeding available balance
+function canAcceptBill(denomination: number): boolean {
+  const ctx = context.value
+  if (!ctx) return false
+  if (ctx.availableBalance <= 0 || ctx.exchangeRate === 0) return true
+  const newFiatCents = ctx.fiatAmount + denomination * 100
+  const grossSats = Math.floor((newFiatCents / 100) * ctx.exchangeRate)
+  const fee = Math.floor(grossSats * ctx.feePercent)
+  return grossSats - fee <= ctx.availableBalance
+}
+
+const balanceLimitReached = computed(() => {
+  if (!context.value || context.value.availableBalance <= 0) return false
+  // Check if even the smallest denomination ($1) would exceed
+  return !canAcceptBill(1)
+})
+
 // Simulate bill insertion (for development)
 function simulateBill(denomination: number) {
   atmStore.insertBill(denomination)
@@ -210,10 +227,26 @@ const isProcessing = computed(() => atmStore.isPayingInvoice || atmStore.isReque
             </div>
           </div>
 
+          <!-- Available balance indicator -->
+          <div v-if="context && context.availableBalance > 0" class="flex justify-between text-sm">
+            <span class="text-muted-foreground">Available:</span>
+            <span :class="balanceLimitReached ? 'text-foreground' : 'text-muted-foreground'">
+              {{ formatSats(context.availableBalance) }} sats
+            </span>
+          </div>
+
           <!-- Bill Acceptor Visual -->
           <div class="rounded-lg border-2 border-dashed border-white/20 p-6 text-center">
-            <div class="mb-2 text-4xl">💵 ➡️ 🏧</div>
-            <p class="text-sm text-muted-foreground">Insert bills into the acceptor</p>
+            <div v-if="balanceLimitReached">
+              <div class="mb-2 text-4xl">💰</div>
+              <p class="text-sm text-muted-foreground">
+                Maximum amount reached — press Done to continue
+              </p>
+            </div>
+            <div v-else>
+              <div class="mb-2 text-4xl">💵 ➡️ 🏧</div>
+              <p class="text-sm text-muted-foreground">Insert bills into the acceptor</p>
+            </div>
           </div>
 
           <!-- Dev: Simulate bill insertion -->
@@ -227,6 +260,7 @@ const isProcessing = computed(() => atmStore.isPayingInvoice || atmStore.isReque
                   variant="outline"
                   size="sm"
                   class="h-8"
+                  :disabled="!canAcceptBill(denom)"
                   @click="simulateBill(denom)"
                 >
                   ${{ denom }}
diff --git a/packages/state-machine/src/machine.ts b/packages/state-machine/src/machine.ts
index 2e0d918..e3260a5 100644
--- a/packages/state-machine/src/machine.ts
+++ b/packages/state-machine/src/machine.ts
@@ -331,6 +331,14 @@ export function createATMMachine(services: Partial<ATMServices> = {}) {
       canRetry: ({ context }) => context.retryCount < 3,
       hasUserNpub: ({ context }) => context.userNpub !== null,
       hasOfferRequest: ({ context }) => context.pendingOfferRequest !== null,
+      billWithinBalance: ({ context, event }) => {
+        if (event.type !== 'BILL_INSERTED') return false
+        if (context.availableBalance <= 0 || context.exchangeRate === 0) return true
+        const newFiatCents = context.fiatAmount + event.denomination * 100
+        const grossSats = Math.floor((newFiatCents / 100) * context.exchangeRate)
+        const fee = Math.floor(grossSats * context.feePercent)
+        return grossSats - fee <= context.availableBalance
+      },
       // Cash-out guards
       hasSelectedAmount: ({ context }) => context.cashOutSelection.length > 0,
       canAddDenomination: ({ context, event }) => {
@@ -392,6 +400,7 @@ export function createATMMachine(services: Partial<ATMServices> = {}) {
           insertingBills: {
             on: {
               BILL_INSERTED: {
+                guard: 'billWithinBalance',
                 actions: ['addBill', 'calculateSats'],
               },
               BILL_REJECTED: {