diff --git a/packages/ERTP/src/amountStore.js b/packages/ERTP/src/amountStore.js
new file mode 100644
index 00000000000..899afa3e39f
--- /dev/null
+++ b/packages/ERTP/src/amountStore.js
@@ -0,0 +1,32 @@
+import { AmountMath } from './amountMath.js';
+
+/**
+ * @template {AssetKind} [K=AssetKind]
+ * @typedef {object} AmountStore
+ * @property {() => Amount<K>} getAmount
+ * @property {(delta: Amount<K>) => void} increment
+ * @property {(delta: Amount<K>) => boolean} decrement
+ */
+
+/**
+ * @template {AssetKind} [K=AssetKind]
+ * @param {object} state
+ * @param {string} key
+ * @returns {AmountStore<K>}
+ */
+export const makeAmountStore = (state, key) => {
+  return harden({
+    getAmount: () => state[key],
+    increment: delta => {
+      state[key] = AmountMath.add(state[key], delta);
+    },
+    decrement: delta => {
+      if (AmountMath.isGTE(state[key], delta)) {
+        state[key] = AmountMath.subtract(state[key], delta);
+        return true;
+      }
+      return false;
+    },
+  });
+};
+harden(makeAmountStore);
diff --git a/packages/ERTP/src/paymentLedger.js b/packages/ERTP/src/paymentLedger.js
index b11ecc925a7..a20f6e4be5e 100644
--- a/packages/ERTP/src/paymentLedger.js
+++ b/packages/ERTP/src/paymentLedger.js
@@ -200,10 +200,6 @@ export const preparePaymentLedger = (
     }
   };
 
-  /** @type {(left: Amount, right: Amount) => Amount} */
-  const add = (left, right) => AmountMath.add(left, right, brand);
-  /** @type {(left: Amount, right: Amount) => Amount} */
-  const subtract = (left, right) => AmountMath.subtract(left, right, brand);
   /** @type {(allegedAmount: Amount) => Amount} */
   const coerce = allegedAmount => AmountMath.coerce(brand, allegedAmount);
   /** @type {(left: Amount, right: Amount) => boolean} */
@@ -239,17 +235,13 @@ export const preparePaymentLedger = (
   /**
    * Used by the purse code to implement purse.deposit
    *
-   * @param {Amount} currentBalance - the current balance of the purse before a
-   *   deposit
-   * @param {(newPurseBalance: Amount) => void} updatePurseBalance - commit the
-   *   purse balance
+   * @param {import('./amountStore.js').AmountStore} balanceStore
    * @param {Payment} srcPayment
    * @param {Pattern} [optAmountShape]
    * @returns {Amount}
    */
   const depositInternal = (
-    currentBalance,
-    updatePurseBalance,
+    balanceStore,
     srcPayment,
     optAmountShape = undefined,
   ) => {
@@ -261,13 +253,12 @@ export const preparePaymentLedger = (
     assertLivePayment(srcPayment);
     const srcPaymentBalance = paymentLedger.get(srcPayment);
     assertAmountConsistent(srcPaymentBalance, optAmountShape);
-    const newPurseBalance = add(srcPaymentBalance, currentBalance);
     try {
       // COMMIT POINT
       // Move the assets in `srcPayment` into this purse, using up the
       // source payment, such that total assets are conserved.
       deletePayment(srcPayment);
-      updatePurseBalance(newPurseBalance);
+      balanceStore.increment(srcPaymentBalance);
     } catch (err) {
       shutdownLedgerWithFailure(err);
       throw err;
@@ -278,30 +269,19 @@ export const preparePaymentLedger = (
   /**
    * Used by the purse code to implement purse.withdraw
    *
-   * @param {Amount} currentBalance - the current balance of the purse before a
-   *   withdrawal
-   * @param {(newPurseBalance: Amount) => void} updatePurseBalance - commit the
-   *   purse balance
+   * @param {import('./amountStore.js').AmountStore} balanceStore
    * @param {Amount} amount - the amount to be withdrawn
    * @param {SetStore<Payment>} recoverySet
    * @returns {Payment}
    */
-  const withdrawInternal = (
-    currentBalance,
-    updatePurseBalance,
-    amount,
-    recoverySet,
-  ) => {
+  const withdrawInternal = (balanceStore, amount, recoverySet) => {
     amount = coerce(amount);
-    AmountMath.isGTE(currentBalance, amount) ||
-      Fail`Withdrawal of ${amount} failed because the purse only contained ${currentBalance}`;
-    const newPurseBalance = subtract(currentBalance, amount);
-
     const payment = makePayment();
+    // COMMIT POINT Move the withdrawn assets from this purse into
+    // payment. Total assets must remain conserved.
+    balanceStore.decrement(amount) ||
+      Fail`Withdrawal of ${amount} failed because the purse only contained ${balanceStore.getAmount()}`;
     try {
-      // COMMIT POINT Move the withdrawn assets from this purse into
-      // payment. Total assets must remain conserved.
-      updatePurseBalance(newPurseBalance);
       initPayment(payment, amount, recoverySet);
     } catch (err) {
       shutdownLedgerWithFailure(err);
diff --git a/packages/ERTP/src/purse.js b/packages/ERTP/src/purse.js
index 74451170fe0..52fc1ca11e3 100644
--- a/packages/ERTP/src/purse.js
+++ b/packages/ERTP/src/purse.js
@@ -2,6 +2,7 @@ import { M } from '@agoric/store';
 import { prepareExoClassKit, makeScalarBigSetStore } from '@agoric/vat-data';
 import { AmountMath } from './amountMath.js';
 import { makeTransientNotifierKit } from './transientNotifier.js';
+import { makeAmountStore } from './amountStore.js';
 
 // TODO `InterfaceGuard` type parameter
 /** @typedef {import('@endo/patterns').InterfaceGuard} InterfaceGuard */
@@ -37,11 +38,6 @@ export const preparePurseKind = (
   // broken across an upgrade.
   const { provideNotifier, update: updateBalance } = makeTransientNotifierKit();
 
-  const updatePurseBalance = (state, newPurseBalance, purse) => {
-    state.currentBalance = newPurseBalance;
-    updateBalance(purse, purse.getCurrentAmount());
-  };
-
   // - This kind is a pair of purse and depositFacet that have a 1:1
   //   correspondence.
   // - They are virtualized together to share a single state record.
@@ -72,28 +68,34 @@ export const preparePurseKind = (
           // PurseI does *not* delay `deposit` until `srcPayment` is fulfulled.
           // See the comments on PurseI.deposit in typeGuards.js
           const { state } = this;
+          const { purse } = this.facets;
+          const balanceStore = makeAmountStore(state, 'currentBalance');
           // Note COMMIT POINT within deposit.
-          return depositInternal(
-            state.currentBalance,
-            newPurseBalance =>
-              updatePurseBalance(state, newPurseBalance, this.facets.purse),
+          const srcPaymentBalance = depositInternal(
+            balanceStore,
             srcPayment,
             optAmountShape,
           );
+          updateBalance(purse, balanceStore.getAmount());
+          return srcPaymentBalance;
         },
         withdraw(amount) {
           const { state } = this;
+          const { purse } = this.facets;
+          const balanceStore = makeAmountStore(state, 'currentBalance');
           // Note COMMIT POINT within withdraw.
-          return withdrawInternal(
-            state.currentBalance,
-            newPurseBalance =>
-              updatePurseBalance(state, newPurseBalance, this.facets.purse),
+          const payment = withdrawInternal(
+            balanceStore,
             amount,
             state.recoverySet,
           );
+          updateBalance(purse, balanceStore.getAmount());
+          return payment;
         },
         getCurrentAmount() {
-          return this.state.currentBalance;
+          const { state } = this;
+          const balanceStore = makeAmountStore(state, 'currentBalance');
+          return balanceStore.getAmount();
         },
         getCurrentAmountNotifier() {
           return provideNotifier(this.facets.purse);