Use a related table of payment records linked to the record that records the sale of this one item. (Don't know structure of your databse, but this is likely a lineitem record filled via a portal to line items on an invoices layout.)
Each time you receive a payment, create a new payment record and record the amount and link it to the correct item sold. A sum calculation can total up the payments so that Sales Amount - Sum ( payments::Amount ) will compute the balance remaining. One fairly simple way to set this up is to use a portal to payments on a layout based on LineItems.
Before you try to set that up, you may want to examine this simple invoicing demo file to see if a set like it uses is what you have or perhaps what you can modify your database to support. It doesn't show recording payments like this, but does show the basic invoicing set up that I recommend you start with before trying to add the ability to record payments like this.