Fix SendMax with no transfer fee

src/test/app/MPToken_test.cpp

@@ -999,6 +999,30 @@ class MPToken_test : public beast::unit_test::suite
             env(pay(bob, carol, MPT(100)), sendmax(MPT(115)));
         }
 
+        // Insufficient SendMax with no transfer fee
+        {
+            Env env{*this, features};
+
+            MPTTester mptAlice(env, alice, {.holders = {&bob, &carol}});
+
+            mptAlice.create(
+                {.ownerCount = 1, .holderCount = 0, .flags = tfMPTCanTransfer});
+
+            // Holders create MPToken
+            mptAlice.authorize({.account = &bob});
+            mptAlice.authorize({.account = &carol});
+            mptAlice.pay(alice, bob, 1'000);
+
+            auto const MPT = mptAlice["MPT"];
+            // SendMax is less than the amount
+            env(pay(bob, carol, MPT(100)),
+                sendmax(MPT(99)),
+                ter(tecPATH_PARTIAL));
+
+            // Payment succeeds if sufficient SendMax is included.
+            env(pay(bob, carol, MPT(100)), sendmax(MPT(100)));
+        }
+
         // Issuer fails trying to send more than the maximum amount allowed
         {
             Env env{*this, features};

src/xrpld/app/tx/detail/Payment.cpp

@@ -686,13 +686,14 @@ applyHelper<MPTIssue>(ApplyContext& ctx, XRPAmount const&, XRPAmount const&)
             isFrozen(ctx.view(), uDstAccountID, mpt))
             return tecLOCKED;
 
-        auto const sendMax(ctx.tx[~sfSendMax]);
-        // If the transfer fee is included then SendMax has to be included
-        // and be large enough to cover the fee. The payment can still fail if
-        // the sender has insufficient funds.
-        if (auto const rate = transferRate(ctx.view(), mpt.getMptID());
-            rate != parityRate &&
-            (!sendMax || multiply(saDstAmount, rate) > *sendMax))
+        auto const sendMax = ctx.tx[~sfSendMax].value_or(saDstAmount);
+        // If SendMax is included then it should be greater or equal to
+        // the send amount plus the transfer fees if applicable.
+        // The payment can still fail if the sender has insufficient funds.
+        auto const amount =
+            multiply(saDstAmount, transferRate(ctx.view(), mpt.getMptID()));
+        if (multiply(saDstAmount, transferRate(ctx.view(), mpt.getMptID())) >
+            sendMax)
             return tecPATH_PARTIAL;
     }