For enable Backorders
To enable backorders, go to:Admin Panel -> System -> Configuration -> Catalog Tab -> Inventory -> Product Stock Options -> Backorders = Allow Qty Below 0 and Notify CustomerEdit Product to Qty = 0 and Stock Availability = In Stock
When you add the particular product to cart, the product is added to cart and you will see the following message:* This product is not available in the requested quantity. 1 of the items will be backordered.You can order the product even if it is out of stock. The product will be delivered to you when it is in stock.