The sales discount button (located just above the F2 button) will allow you to apply a discount to all SALE items on the screen. Don't worry, it will not apply a discount to an item that already has a discount applied to it. So I could put an item on the screen and apply a 10% discount, then put another item on the screen and apply a 5% discount, and it would not affect the item with the 10% discount. To reverse a discount, highlight the item with F9 or the mouse, and press enter twice, and the price will revert to the normal price.