>You may not have meant it exactly this way, but it appears you are suggesting that the payments apply "in order" against the oldest outstanding invoices. In fact, the customer needs to select how any credits (regardless of how they are stored in the database) are applied against invoices. Also, the creation of credit invoices is quite common, so I wouldn't call that odd. And there are complexities here:
>
>Payment of $1000
>
>Inv. #1 - $500 - $400 applied
>Inv. #2 - $500 - $600 applied
>
>So the entire $1000 is applied, but there is a underpayment on one invoice and an overpayment on another. They have a $0 balance overall, but you still have to determine and address the situation. There is nothing left to apply on the payment, but there is a $100 credit invoice that can be applied to wipe out the under payment on invoice 1. The balance still remains at zero, but now all invoices are marked with a zero balance.

Every time I think of this whole thing as "rubbish, what does the money know - debt is a debt is a debt, and payment is a payment is a payment, what does it matter which is paid?", I have to pinch myself and remember which legal system are we talking about.

There are debts and there are debts, right? The contractual conditions on them may be different, and it does matter which one is paid and specially when - because being late on one may not have the same consequences as being late on another.

Is that the reason for this desire to do strict binding? Just trying to understand.