We presently have no way to support what you describe. However, if you have Inventory Control enabled, the cart will prevent buyers from ordering an item which is out of stock, and will prevent them from ordering more units than your inventory stock figures say you have in stock -- e.g., if they Add to Cart an item and try to change its quantity to 10, but you only have 7 left, when they Update Cart it will show only 7 units in their cart order.