We want to create an invoice with a negative inventory line item so customer can swap out parts/products. For example if the customer wants to buy a product and swap out a part of it, say a standard battery for high capacity battery, we want to simply enter -2 QTY for the standard battery and then +2 for the high capacity. This way the inventory is kept accurate and we don't need to create a credit note or do an inventory adjustment. There's no need to create a credit note because the customer is not "returning" anything and adjusting inventory all the time is wasteful. The "swap" needs to appear on the invoice so the customer knows they are getting what they requested.