diff --git a/apps/machine/src/stores/atm.ts b/apps/machine/src/stores/atm.ts
index 7955f9f..f6c9380 100644
--- a/apps/machine/src/stores/atm.ts
+++ b/apps/machine/src/stores/atm.ts
@@ -28,13 +28,16 @@ const isElectron = typeof window !== 'undefined' && window.electronAPI !== undef
 
 /**
  * Compute fee in sats from context.
- * Cash-in: customer gets net (gross - fee). Cash-out: customer pays gross + fee.
+ *
+ * principalSats = market-rate sats for the fiat amount (fiat × exchangeRate, no commission).
+ * Cash-in:  on-wire satsAmount = principalSats - feeSats (customer receives less than principal)
+ * Cash-out: on-wire satsAmount = principalSats + feeSats (customer pays more than principal)
  */
 function computeFeeSats(ctx: ATMContext, isCashIn: boolean): number {
-  const grossSats = ctx.exchangeRate > 0 ? Math.floor((ctx.fiatCents / 100) * ctx.exchangeRate) : 0
+  const principalSats = ctx.exchangeRate > 0 ? Math.floor((ctx.fiatCents / 100) * ctx.exchangeRate) : 0
   const feeSats = isCashIn
-    ? grossSats - ctx.satsAmount // cash-in: customer gets net
-    : ctx.satsAmount - grossSats // cash-out: customer pays gross + fee
+    ? principalSats - ctx.satsAmount // cash-in: customer receives less than principal
+    : ctx.satsAmount - principalSats // cash-out: customer pays more than principal
   return Math.max(0, feeSats)
 }
 
@@ -850,9 +853,9 @@ export const useAtmStore = defineStore('atm', () => {
           // Calculate what the new sats amount would be
           const newFiatCents = ctx.fiatCents + denomination * 100
           const newFiatUnits = newFiatCents / 100
-          const grossSats = Math.floor(newFiatUnits * ctx.exchangeRate)
-          const fee = Math.floor(grossSats * ctx.feePercent)
-          const newSatsAmount = grossSats - fee
+          const principalSats = Math.floor(newFiatUnits * ctx.exchangeRate)
+          const fee = Math.floor(principalSats * ctx.feePercent)
+          const newSatsAmount = principalSats - fee
 
           // Check against available balance
           if (newSatsAmount > ctx.availableBalance) {
@@ -1106,9 +1109,9 @@ export const useAtmStore = defineStore('atm', () => {
         // Calculate if this bill would exceed available balance
         const newFiatCents = ctx.fiatCents + denomination * 100
         const newFiatUnits = newFiatCents / 100
-        const grossSats = Math.floor(newFiatUnits * ctx.exchangeRate)
-        const fee = Math.floor(grossSats * ctx.feePercent)
-        const newSatsAmount = grossSats - fee
+        const principalSats = Math.floor(newFiatUnits * ctx.exchangeRate)
+        const fee = Math.floor(principalSats * ctx.feePercent)
+        const newSatsAmount = principalSats - fee
 
         if (newSatsAmount > ctx.availableBalance) {
           console.log(
diff --git a/apps/machine/src/views/CashInView.vue b/apps/machine/src/views/CashInView.vue
index 7956ca7..2a150b2 100644
--- a/apps/machine/src/views/CashInView.vue
+++ b/apps/machine/src/views/CashInView.vue
@@ -120,9 +120,9 @@ function canAcceptBill(denomination: number): boolean {
   if (!ctx) return false
   if (ctx.availableBalance <= 0 || ctx.exchangeRate === 0) return true
   const newFiatCents = ctx.fiatCents + denomination * 100
-  const grossSats = Math.floor((newFiatCents / 100) * ctx.exchangeRate)
-  const fee = Math.floor(grossSats * ctx.feePercent)
-  return grossSats - fee <= ctx.availableBalance
+  const principalSats = Math.floor((newFiatCents / 100) * ctx.exchangeRate)
+  const fee = Math.floor(principalSats * ctx.feePercent)
+  return principalSats - fee <= ctx.availableBalance
 }
 
 const balanceLimitReached = computed(() => {
diff --git a/packages/state-machine/src/machine.ts b/packages/state-machine/src/machine.ts
index ebb3916..828eb39 100644
--- a/packages/state-machine/src/machine.ts
+++ b/packages/state-machine/src/machine.ts
@@ -189,9 +189,9 @@ export function createATMMachine(
         satsAmount: ({ context }) => {
           if (context.exchangeRate === 0) return 0
           const fiatDollars = context.fiatCents / 100
-          const grossSats = Math.floor(fiatDollars * context.exchangeRate)
-          const fee = Math.floor(grossSats * context.feePercent)
-          return grossSats - fee
+          const principalSats = Math.floor(fiatDollars * context.exchangeRate)
+          const fee = Math.floor(principalSats * context.feePercent)
+          return principalSats - fee
         },
       }),
       calculateDispenseAmounts: assign({
@@ -324,10 +324,10 @@ export function createATMMachine(
         satsAmount: ({ context }) => {
           if (context.exchangeRate === 0) return 0
           const fiatDollars = context.fiatCents / 100
-          const grossSats = Math.floor(fiatDollars * context.exchangeRate)
+          const principalSats = Math.floor(fiatDollars * context.exchangeRate)
           // For cash-out, user pays the sats, so fee is added
-          const fee = Math.floor(grossSats * context.feePercent)
-          return grossSats + fee
+          const fee = Math.floor(principalSats * context.feePercent)
+          return principalSats + fee
         },
       }),
       calculateDispenseFromSelection: assign({
@@ -372,9 +372,9 @@ export function createATMMachine(
         // cash we can't cover with sats
         if (context.availableBalance <= 0 || context.exchangeRate === 0) return false
         const newFiatCents = context.fiatCents + event.denomination * 100
-        const grossSats = Math.floor((newFiatCents / 100) * context.exchangeRate)
-        const fee = Math.floor(grossSats * context.feePercent)
-        return grossSats - fee <= context.availableBalance
+        const principalSats = Math.floor((newFiatCents / 100) * context.exchangeRate)
+        const fee = Math.floor(principalSats * context.feePercent)
+        return principalSats - fee <= context.availableBalance
       },
       // Cash-out guards
       hasSelectedAmount: ({ context }) => context.cashOutSelection.length > 0,