When a customer exchange takes place we have to raise a credit for the returned item and an invoice for the replacement.  This disassociates the 2 transactions.

It would be useful to have a function whereby you add the returned item as a negative quantity to the invoice for the replacement and end up with a debit or credit value that is posted to the account.  This is then clear to the client what was returned and the replacement and also when looking at historical data internally will be associated with each other in Sage 

This would be similar to an exchange in a retail store.