Add payment differences input box at customer payment entry screen

At the customer payment entry screen there is a handy "amount of discount" input box. Maybe it's an idea to add also a "payment differences" input box.

If EUR 100 is received from a customer, and there is only EUR 99 of invoices to allocate, then it is wishful to add the EUR 1 as payment differences in the P&L. It would be easy if this can be done in the same customer payment entry screen where the total amount is recorded.