Legal Tender: A Definition
31 USC § 5103. Legal Tender
United States coins and currency (including Federal reserve notes and circulating notes of Federal reserve banks and national banks) are legal tender for all debts, public charges, taxes, and dues. Foreign gold or silver coins are not legal tender for debts.
However, there is no Federal statute which mandates that private businesses must accept cash as a form of payment. Private businesses are free to develop their own policies on whether or not to accept cash unless there is a State law which says otherwise.