There are several considerations when a Merchant needs to support splitting a transaction across multiple tenders. InterPayments calls this “multi-pay.”
The Amount field on a Transaction Fee call is the value which is destined for a particular credit capture authorization and capture. When supporting multi-pay, this would be some portion of the overall transaction total.
The TotalAmount field on a Transaction Fee call is the overall transaction total. It is used primarily to support Merchant Business Rules.
There are only a few restrictions on the Amount and TotalAmount fields. For example, validation that the Amount is less than the TotalAmount occurs but no validation that all multi-pay totals add up to the TotalAmount.
Management of the mTxId has different complexities when using multi-pay. InterPayments gives precedence to the sTxId attribute of Transaction Fee calls (including sale) so you must consider the relevance of identifying the record to update if using the same mTxId across different multi-pay transactions. It is less ambiguous for reporting and record identification of a unique mTxId is used for each tender of a multi-pay transaction.