When an item is supply free than first thing you would need to do is to set item to not manage stock. You can do this by going to Products and clicking on a variant.
If the Promotional item is inclusive within an oder, then simply add them in Sales Order with 100% discount applied.
If the promotional item is on its own, then you can deduct stock level by going into Stock Control and Doing a stock adjustment with Promotion as a reason.
Hope this answered your question and have a great day!