I recently set up an application that does something similar. What you need
to do is store with each purchase the payment type and amount. Each customer
should have a field showing the current outstanding balance. Finally, you
need another table to record customer payments. Here's how it works:

When a customer makes a purchase, if any amount is paid at the time of
purchase an entry is made into the Payment table. If the exact purchase
amount is not paid (customer paid either more or less than the total), the
customer's Outstanding Balance field is updated with the unpaid (overpaid)
amount. Any time the customer makes a payment, either with or without a
purchase, an entry is made in the Payment table and that amount is added
to/subtracted from the Outstanding Balance.

The drawback to this is that specific payments are not linked to specific
purchases. For my purpose, this wasn't a problem. Linking specific payments
to specific purchases is more complicated, because you have to look at each
purchase in order and apply payments to each one until it is paid off. You'd
need a table storing the payment amount and which purchase it was applied
to.

Carl Rapson