Is Product Inventory Respected?
The product variant's inventory policy is respected when redeeming a promotion. For example, if the product has no stock available but is marked Continue selling when out of stock, the promotional product will be added to the order. However, if a product is out of stock and Continue selling when out of stock is disabled, the promotion will not be added.
There is currently no option to override the native Shopify inventory behavior. If you're having issues with inventory and promotions let us know.