Customer want his tip back a week later. is this legal?

last thursday i was waiting on several large groups in the bar where i work. our boss is a strong supporter of adding gratuity to the bill when we can and has even said "always add gratuity. and sometimes they don't notice the grat. and will tip you double." i find this to be sneaky and dishonest and always underline or circle the gratuity in addition to it being clearly printed on the bill. i was told that one of my tables were really good tippers, so when they left me an additional fifty bucks on top of the gratuity i thought nothing of it. now, a week later, the customer is saying that he didn't realize that the gratuity was already on the bill and wants the fifty bucks back. my boss is saying that i now owe him fifty dollars. is this legal? i mean, fifty bucks wont make or break me, but what happens in the future when it is a two hundred dollar tip that someone regrets leaving me? the grat was clearly printed, and underlined. and he signed the credit card slip above the line that says "i agree to all above charges." i feel taken advantage of. it would be one thing if it was one or two hours after the fact... but a week later? that money is already in my budget, going to bills. i feel like if my boss wants to take "the customer is always right" route, then that is a business decision that he is voluntarily making and should not involve me. am i wrong about this? am i protected legally in any way?