Can you offer a price match for purchases up to 30 days before via store credit? While calculating items going on sale you can run an estimates according to last 30 days of sales on what you may end up giving credit to on the previous customers based on % of customers that actually take advantageous of it. Not all will be bothered with even doing it.

Why would it not be legal? The employee elects withholding amounts and can change it anytime throughout the year depending on their tax situations. If employee wants to be exempt this pay period have them fill out a new W4 and then another one when they need taxes taken out.