I just came up with an semi-unrelated epiphany reading these comments. Use an indelible pen and write "Dirty Kike" or "Nigger" on the back of $20 bills. This is not illegal, as only defacing money is illegal if you are trying to present the denomination as something else.
Use a pen to cross it slightly out or put a blue box on it with "End Hate" written or some gay shit for plausible deniability if you live in a high nigger zone.