How do you process a trade-in in NAV

We have two customers who accept trade-ins that offset a purchase of a new item. NAV will process a sales order with a new item with a quantity of 1 and a trade-in with a quantity of -1 as long as the amount of the new item is greater than the amount of credit for the trade-in. If you ship and invoice the new item and later try to ship and invoice the negative quantity item (trade-in), you get an error that "Amount must be positive in General Journal Line"

How do others handle this problem.