Can anyone provide any insight into the issue behind this error?  Out of the hundreds of these type transactions my software inserts each day, I see this error on one or two.  I'm not sure if the problem is with this transaction or with the credit memo being referenced.  I can query the referenced credit memo manually and verify that it does exist and it does have credit remaining.

2010-02-10 20:04:33 QODBC Ver:  8.00.00.237 *********************************************************************************************************************
IsAService: False
SQL Statement: INSERT INTO ReceivePaymentLine ( CustomerRefListID,TxnDate,AppliedToTxnTxnID,AppliedToTxnSetCreditCreditTxnID,AppliedToTxnSetCreditAppliedAmount,FQSaveToCache ) VALUES  (  ?, ?, ?, ?, ?, ? )
3173 - The related credit transaction transaction object specified by the ID F461B-1265801454 was deleted or modified.
Input XML:
<?xml version="1.0" encoding="ISO-8859-1"?>
<?qbxml version="6.0"?>
<QBXML>
    <QBXMLMsgsRq onError = "continueOnError" responseData = "includeAll">
        <ReceivePaymentAddRq requestID = "1">
            <ReceivePaymentAdd defMacro = "TxnID:C5A03C7D-8AE0-4D14-98A1-1A399AEAAFE8">
                <CustomerRef>
                    <ListID>800001AF-1175537453</ListID>
                </CustomerRef>
                <TxnDate>2009-12-22</TxnDate>
                <AppliedToTxnAdd>
                    <TxnID>E4D8A-1260469562</TxnID>
                    <SetCredit>
                        <CreditTxnID>F461B-1265801454</CreditTxnID>
                        <AppliedAmount>5.00</AppliedAmount>
                    </SetCredit>
                </AppliedToTxnAdd>
            </ReceivePaymentAdd>
        </ReceivePaymentAddRq>
    </QBXMLMsgsRq>
</QBXML>
Output XML:
<?xml version="1.0" ?>
<QBXML>
    <QBXMLMsgsRs>
        <ReceivePaymentAddRs requestID="1" statusCode="3173" statusSeverity="Error" statusMessage="The related credit transaction transaction object specified by the ID F461B-1265801454 was deleted or modified." />
    </QBXMLMsgsRs>
</QBXML>

The normal format to apply a overpayment is:-

TO APPLY A PAYMENT CREDIT
INSERT INTO ReceivePaymentLine (CustomerRefListID, ARAccountRefListID,
AppliedToTxnTxnID, AppliedToTxnPaymentAmount, AppliedToTxnSetCreditCreditTxnID,
AppliedToTxnSetCreditAppliedAmount) VALUES ('AE0000-1197765289',
'40000-933270541','5C2C-1197773093', 500.00, '5C29-1197772163', 500.00)

By the sounds of things it's possible that the related credit transaction transaction object specified by the ID F461B-1265801454 was modified by either another QuickBooks user or another transaction. I would check the SDK Messages Log for further clues.