I think it might depend on who owns your restaurant (an individual under a franchise agreement or the chain itself) and who is docking your pay, etc. If it is your "boss" I imagine they can make up whatever silly rules they want. Since you're salaried instead of per hr. wage worker, there's not as much protection on what they can and cannot pay you. Do you get bonuses for when your sales go up? If you only get punitive adjustments like this one, I'd look for a better place to work.