Delivery Note Return failure

  • Customer has recevied an order made up of 5 lines of items.
  • Customer is on account so has not paid for order yet.
  • Customer has returned one line of items.
  • I create a return delivery note to reintroduce stock to shelf
  • Return Delivery Note requests serial numbers
  • Inputted serial numbers changes quantity from a negative value to a positive value
  • Return DN won’t submit with a postive value or without serial numbers.

How can I resolve?
Have I approached this wrong?