I have a customer who has an existing credit balance with me. We've agreed that future work will be paid at half my normal rate via direct funds, with the other half applied against that balance until it is satisfied. I'd like these 'credit' line items to display on the individual invoices if possible.

How would I best set all this up in QuickBooks so that I may properly account for both new work and the credit balance, so that half of the billed subtotal shows on the invoice as having been applied to the balance? (Ideally, I'd be able to send him a monthly statement showing the current state of the credit—but if that's not possible I can live without it.)