Legal tender is currency that cannot legally be refused in payment of debt. The Coinage Act of 1965, specifically Section 31 U.S.C. 5103, defines legal tender as "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."
This statute means that all United States money as identified above are a valid and legal offer of payment for debts when tendered to a creditor. There is, however, no federal statute requiring a private business, a person or an organization to accept currency or coins as for payment for goods and/or services. Private businesses may adopt their own policies on whether or not to accept cash as long it doesn't violate state law. For example, a business may refuses to accept payment in pennies or large denomination bills as a matter of policy.