I'm reasonably certain they won't do it retroactively because of their exchange policy:
If you buy a pair of shoes during a sale and it's the wrong size, they won't let you exchange them for a different size. You buy a new pair of shoes in the right size at whatever the current price is and then return the old pair.
If your size is still in stock, buy them at the 20% off and return the ones you bought yesterday when they arrive.