fix: use typed amount for Money Account max source calculations

packages/transaction-pay-controller/CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Use the typed required amount for Money Account `isMaxAmount` source calculations instead of the pay token's on-chain balance, so Max deposits funded from the money account (e.g. Send to Perps) keep the full withdrawable total (mUSD + vmUSD) and can use `EXACT_INPUT` ([#9707](https://github.com/MetaMask/core/pull/9707))
 - Fix Relay quote validation ([#9723](https://github.com/MetaMask/core/pull/9723))
   - Keep the quote when validation fails with reason `insufficient-source-balance`, while still surfacing `quoteError`; all other validation-failure reasons continue to remove the quote.
   - Exclude a zero `gas` value from the simulated transaction.

packages/transaction-pay-controller/src/utils/source-amounts.test.ts

@@ -1,6 +1,10 @@
 import { TransactionType } from '@metamask/transaction-controller';
 
-import { ARBITRUM_USDC_ADDRESS, CHAIN_ID_ARBITRUM } from '../constants.js';
+import {
+  ARBITRUM_USDC_ADDRESS,
+  CHAIN_ID_ARBITRUM,
+  PaymentOverride,
+} from '../constants.js';
 import { TransactionPayStrategy } from '../index.js';
 import type { TransactionPaymentToken } from '../index.js';
 import { getMessengerMock } from '../tests/messenger-mock.js';
@@ -264,6 +268,42 @@ describe('Source Amounts Utils', () => {
       ]);
     });
 
+    it('uses fiat-derived source amount for MoneyAccount max instead of payment token balance', () => {
+      // Money account withdrawable (mUSD + vmUSD) is reflected in the typed
+      // required token amount. The pay token's on-chain balance is only the
+      // un-vaulted mUSD portion and must not collapse Max.
+      const transactionData: TransactionData = {
+        isLoading: false,
+        isMaxAmount: true,
+        paymentOverride: PaymentOverride.MoneyAccount,
+        paymentToken: {
+          ...PAYMENT_TOKEN_MOCK,
+          // Bare on-chain mUSD — much smaller than the typed max.
+          balanceHuman: '0.62',
+          balanceRaw: '620000',
+          balanceUsd: '0.62',
+        },
+        tokens: [
+          {
+            ...TRANSACTION_TOKEN_MOCK,
+            // Full withdrawable max typed by the client ($6.00 USD).
+            amountUsd: '6.0',
+          },
+        ],
+      };
+
+      updateSourceAmounts(TRANSACTION_ID_MOCK, transactionData, messenger);
+
+      // usdRate mock is 3.0 → source human = 6 / 3 = 2, raw = 2 * 10^6.
+      expect(transactionData.sourceAmounts).toStrictEqual([
+        {
+          sourceAmountHuman: '2',
+          sourceAmountRaw: '2000000',
+          targetTokenAddress: TRANSACTION_TOKEN_MOCK.address,
+        },
+      ]);
+    });
+
     it('does nothing if no payment token', () => {
       const transactionData: TransactionData = {
         isLoading: false,

packages/transaction-pay-controller/src/utils/source-amounts.ts

@@ -8,6 +8,7 @@ import { BigNumber } from 'bignumber.js';
 import {
   ARBITRUM_USDC_ADDRESS,
   CHAIN_ID_ARBITRUM,
+  PaymentOverride,
   PERPS_DEPOSIT_TYPES,
 } from '../constants.js';
 import type {
@@ -42,7 +43,8 @@ export function updateSourceAmounts(
     return;
   }
 
-  const { isMaxAmount, isPostQuote, paymentToken, tokens } = transactionData;
+  const { isMaxAmount, isPostQuote, paymentOverride, paymentToken, tokens } =
+    transactionData;
 
   if (!tokens.length || !paymentToken) {
     return;
@@ -75,6 +77,7 @@ export function updateSourceAmounts(
         transactionId,
         isMaxAmount ?? false,
         isQuoteRequired,
+        paymentOverride,
       ),
     )
     .filter(Boolean) as TransactionPaySourceAmount[];
@@ -148,6 +151,7 @@ function calculatePostQuoteSourceAmounts(
  * @param transactionId - ID of the transaction.
  * @param isMaxAmount - Whether the transaction is a maximum amount transaction.
  * @param isQuoteRequired - When true, a quote is always fetched even when source and target tokens are identical.
+ * @param paymentOverride - Optional payment source override for the transaction.
  * @returns The source amount or undefined if calculation failed.
  */
 function calculateSourceAmount(
@@ -157,6 +161,7 @@ function calculateSourceAmount(
   transactionId: string,
   isMaxAmount: boolean,
   isQuoteRequired?: boolean,
+  paymentOverride?: PaymentOverride,
 ): TransactionPaySourceAmount | undefined {
   const paymentTokenFiatRate = getTokenFiatRate(
     messenger,
@@ -208,7 +213,12 @@ function calculateSourceAmount(
     return undefined;
   }
 
-  if (isMaxAmount) {
+  // Money account Max must not use the pay token's on-chain balance. That
+  // balance is only un-vaulted mUSD, while the typed required amount already
+  // reflects the full withdrawable total (mUSD + vmUSD). Using the typed
+  // fiat-derived source keeps isMaxAmount=true (EXACT_INPUT) correct for
+  // deposits funded from the money account (e.g. Send to Perps).
+  if (isMaxAmount && paymentOverride !== PaymentOverride.MoneyAccount) {
     return {
       sourceAmountHuman: paymentToken.balanceHuman,
       sourceAmountRaw: paymentToken.balanceRaw,