It would have been pointless, but you could have pulled out a dollar bill and pointed to the fine print that says "This currency is legal tender for all debts, public and private". Which at its root means that it is illegal to refuse American currency offered in payment of a debt.
I made the mistake of thinking I was dealing with a rational person. I had left my wallet at home, but it wasn't like I dumped a hundred pennies down, either--quarters, dimes. I mean, the guy had NO other customers, not a single person in the area but me. Is the life of someone sitting on his ass in a booth THAT complicated that he can't count change for someone who pays his salary? I mean, I've never seen a sign saying "paper money ONLY" over one of those booths.