"My employer takes a credit card fee out at 2.5% of my credit tips."
That's sounds reasonable to me. They only get to keep 97.5% of the bar tab, because the credit card company gets the other 2.5%. The same should apply to you. It may be petty, but it's reasonable and probably legal (I'm not a lawyer). They only received 97.5% of the tip, but you want them to pay you 100%.
