This comes up a lot. Many like to perform negative inventory transactions directly on a sales order. Having to create a separate order type for the return takes additional time.
Example today where they sell a base item, and the customer essential will upgrade from a standard part. Call it a 50' hose but instead will buy a 100' hose. The 50' hose is still a value and can be sold later from stock. So the order should look like this.
Qty Item Price
1 Base Item $1,000
-1 50' Hose $0
1 100' Hose $50
So negative qty's can help out in many ways.