Ernst may be right if your link is a many-to-many link from Receivables to Payments, but if the link is a one-to-many link from Payments to Receivables, a pair of Triggers is needed, one to execute when a payment is first saved & another when the payment is later updated
Let me know which kind of link you have, and if it is the one-to-many link and if you know how to do this or if you know JavaScript at all. If not, wait for my next posting (some hours from now, as I need to sleep now) in which I will provide the solution to you.