Hello,
I would like to underline that this is not a problem, but a standard, and in my opinion rightful behaviour of SAP: in a dowpayment process, when you create the invoice document, you should deduct only an amount less or equal to the invoice amount. And in the following invoices you will deduct the remaining downpayment amounts still open.
Also under al legal and fiscal point of view, I guess how would you print the invoice? With a minus value?
Best regards,
Andrea