Yes, in the Open Invoice file there is a field "Payments Today"  or "Paid Today", can't remember which, there is probably a value in that field. You would use DFDM to go into the file and clear that field.

File - Run - ARWAR4.

(Oops... that would be for AR invoices.  I just notice the OP's tags stating these are AP invoices).

Data File Display and Maintenance (under Library Master - Utilities). You're basically editing the raw back-end data, which can obviously cause problems if you don't know what you're doing.