Custom Checkout Original Price

in product page we need to show a sale price to push people get the first offer so we have two prices: sale and main price. in checkout offer page there should be an option to show main price or sale price so if we offer buy 2 we have option to add extra discount on the sale price and show not only get sale price but by buying 2 pairs you will get even more discount. but in current version if we want to give extra discount for the second offer we should calculate from the main price not applying discount on sale price so the final price is not that much accurate.