Customer returns should show as a negative quantity

It would be helpful to be able to see the quantity of parts returned by a customer in the Total Promised field. In the example, 31 pieces were shipped. However, the customer returned 11 pieces, which were reworked and shipped back. When they were shipped back to the customer, the Total Promised kept increasing. It now shows that 31 pieces have been delivered, when the true quantity shipped is only 20.