example

i have a sales order with 2 lines

1 line for a botlle  with quantity 1

1 line for return bottle with quantity -1

when i save the quotation, the demanded quantity is automaticly changed to 0

1 - 1 = 0

this results in a picking move that is automaticly cancelled becasue the demand quantity is 0

but this has to stay a 1 or 2 ( depending on how many ordered) because we have to send these bottles to the customer and wait for return

can someone help?