As the title suggests, I am struggling to both aggregate a form (Form A) that collects payments made against specific invoices (the matching value between the two forms or reference field is the PO#) and then updating a field on Form B that reflects the outstanding balance (or Total minus Sum of Form A payments). Any assistance would be GREATLY appreciated. Thank you.

In theory, the outcome should be as follows. Form A captures payments made against invoices. Form B is updated when Form A has an applicable payment submitted and updates the Outstanding Balance field/value in Form B. Form B is used to track the invoice itself and includes the total for the invoice, any deposits, and the outstanding balance owed. Form A only tracks payments made by customers.