Sorry I still don’t understand your question. Hope this is will help.
When a product is out of stock, POS will give an out-of-stock warning message when selling. You can still sell the product. In most case, if the customer has the item physical on hand, the quantity number in POS system is probably wrong. The quantity should be manually adjusted or though inventory count. If you have layaway or web orders, the system will put item on-hold (A column other than QtyOnhand), and you should put the item physically aside(on hold).