Prism makes it easy to do a merchandise exchange. An exchange consists of two separate events: a return and a purchase. The replacement item is typically the same sku as the return item.
When an item is listed as a return item and then as a sale item, Prism will show both line items on the transaction (instead of consolidating the quantities into a single line item). This makes it easier to tell that the transaction is a straight exchange.
List the item being returned and change the Item Type to "Return." Reference the original purchase transaction, if needed. The return item is now listed on the transaction.
List the item a second time. By default, the Item Type is set to "Sale". The item is now listed on the transaction twice: once as a return item and once as a sale item.
The screenshot below shows a transaction with the same item listed twice, once as a sale item and once as a return item.