You must go into parts point of sale > enter the part number and select minus in the quantity field for as many needed to be refunded.
Once this is done you must click on the add button when all the information for the parts are correct (quantity and price).
A box will appear for override instructions.
You must then enter the override keyword (4 digit number found in setup > general setup)
Once this has been done you must select the customer it will be credited to via the customer (F8) button.
This will then be complete - this is simply raising a credit without an invoice ref.