client overpay

Hi there,i have a coulpe of clients who occasionally overpay there invoices(not the worst problem to have) However i can find no way of matching there payment to there invoice while leaving them in credit. ie. one client had an invoice for $70.00 and paid $80.00 into into my account. However i try to mark the payment against the invoice it just wont do it. And says 100% of the payment must be alloctated. I tried  marking $70.00 against the bill,and the remaining $10.00 just into my account. But that dosent work either. Any ideas on how i can make this work? cheers