Calculate Balance due in a form from other tables in a database

[marks]

I have a database with an invoice form and need to get the balance due field
to auto calculate the difference between many payments already made. Previous
payments are stored in another table/form within the database. I need to
somehow find the product #, which is entered in this form while creating the
invoice, and auto calculate the balance due field.

Thanks for any help you can provide,
Mark