Change Payment Schedule Basis

It would be helpful to have the option to change the basis the payment schedule is based on. We're trying to implement payment credits, which reduce the field "total_customer_payment," but the credits don't reduce the field "price" (and/or whatever field the payment schedule calculations are based on). Currently, if we include payment credits, we have to manually change the payment schedule table because the payments otherwise don't add up to the total due from the customer.