Invoices and Payments

[Papabjoe]

I'm setting out to design a billing system that will show me a list of related "open" invoices for a particular client. I'm having trouble visualizing a payment system where I can enter a single payment amount, and still controll how that payment is distributed to various invoices. For instance, if someone makes a partial payment how do I record that so that the payment is applied, but does not show up in the future as a credit. I just want the balance to be adjusted. Any suggestions?