When you enter a Credit Return or an Exchange service item entry then move to the Lines panel and highlight that entry in the service item list box, you will see the item to be returned in the lines grid.  This allows you to change the price you wish to refund to the customer and enter a comment.  There are no other changes allowed to this line.  When the service item entry is complete and a credit memo is created, this line will be copied to the lines of that credit memo.