There is a loophole, although you cant gain money, only store credit. I once bought an item on sale and returned it when the price had gone back up, although i had misplaced my receipt. They refunded me the full price of the item without question as a credit note, and even though i told them i paid less, they were still happy to refund the higher value because neither the store or myself could remember the exact sale price (Was a pair of jeans around £30-£45 quid).