ReceivePayment

In a ReceivePayment message, each  AppliedToTxnRet aggregate represents the transaction to which this part of the payment is being applied. Four types of transactions can receive payments in QuickBooks: invoices, statement charges (though not through the SDK), general journal debits, and checks. (For example, a customer’s job might receive a payment reimbursing a check that was written for an expense for that customer’s job.)

Or use:

IsAutoApply
If IsAutoApply is true, QuickBooks applies TotalAmount to any outstanding transaction that exactly matches TotalAmount. If no exact match is found, the payment is applied to the customer job’s outstanding transactions, beginning with the oldest one.

How to use IsAutoApply in LetSync Desktop. You will have to set the EditSequence = applyall on the record that you want to use it

• AppliedToTxnRet

• CCTII ->CreditCardTxnInputInfo
• CCTRI ->CreditCardTxnResultInfo

• Adding new record: EditSequence=’ApplyAll’

If ApplyAll is set, QuickBooks applies TotalAmount to any outstanding transaction that exactly matches the total amount. If no exact match is found, the payment is applied to the customer job’s outstanding transactions, beginning with the oldest one.
If IsAutoApply isn’t set, QuickBooks receives the payment but does not apply it to any outstanding transaction. Instead, the amount will appear as a credit on the customer job’s next transaction.
Or you can set:

AppliedToTxnRet.PARENT_IDKEY -> ReceivePayment.TxnID
AppliedToTxnRet.TxnID -> Invoice.TxnID