A/R credits associated with a specific item #

My client needs reports showing credits granted to customers based on item…for example, they sell 16 widgets for $160 ($10 each)…two months later, their customer submits a payment of $60 and takes a credit for $100 because they advertised the product in their circular. Our client wants to see a report in which they see the original sale of 16 widgets (Item 123, for example) @ $160 followed by a line with a “smart” item number (such as A123) indicating that a credit was taken for 10 widgets @ $100 due to advertising. Credits are also granted for other reasons, and they need a smart number to indicate the reason for the credits, such as A123 for advertising, V123 for volume, etc. Any suggestions as to how I can create “fake” items to reflect these memo entries for reporting purposes?