There are two methods that this task can be completed by: 
  1. Enter all items into a standard salve
  2. Change the quantity of the item to be returned to a negative number. For example, if 2 items are being returned, enter -2.
  
OR
 
  1. Ring up the first item being sold
  2. Press CTRL+F4 to enter Return mode
  3. Enter the item being returned. Note that the quantity automatically rings up as -1.
  4. Repeat entering items until all return items have been entered
  5. Press CTRL+F4 to exit Return mode
  6. Enter all other items to be sold. Ensure these items have a positive quantity.
  7. Tender the sale