The trick here is Not to use the original inventory item on the Credit memo: "For example, if we have a customer return and the goods are damaged or undesirable, we'd like to debit the sales rep's gross profit number."
You want to list a Value for the activity, as a Noninventory or Other Charge Type of item, such as "RMA only" and not have this returned to stock first, to be adjusted afterwards. You want the credit memo to show the Rep, because a credit memo is the reversal of the sale. Then, you have the time to declare if the item goes back into stock, needs to be repaired first, or is being disposed of. And think of the inventory sale item that never arrived or was so damaged, no one should spend money returning it to you at all. That's why you only list the Inventory Item on the Credit memo when you actually got it back and Into the warehouse on hand ready to sell, again.