Freebies with a sale, you should use two item lines

widget, qty 20, $$$
widget, qty 3,   $0.00

When you use a negative number for qty, that puts the item back in inventory and pulls the average cost out of COGS

But to track promotional items, given away over the counter so to speak, use inventory adjust, set the adjusting account to promotional expense, and lower the qty - then the cost of the items hit the promotional expense account as they should.

There is no alias function in QB inventory, so you can not make a giveaway item that will affect qty on hand, and you can not just assign a cost to any item with out that cost coming from some account.