"this note is legal tender for all debts, public and private" printed on every dollar(s) bill in circulation should settle the argument. Cash should always be a

"this note is legal tender for all debts, public and private" printed on every dollar(s) bill in circulation should settle the argument. Cash should always be an option. It's the law. If that law is invalid, how can I know that the law that says "don't pay for more than X in cash" is valid?