“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.”The consequences of this are that, while US currency and coin (and high powered money as bank reserves) is legal tender and cannot be refused as payment to discharge debt, it is not illegal for the private sector to accept debt instruments, such as cheques, bills of exchange, promissory notes, drafts and bank money (or fractional reserve demand deposit money) as payment, before final clearing takes place.
31 U.S.C. § 5103, http://www.federalreserve.gov/faqs/currency_12772.htm
And another consequence is that private checks, promissory notes, bills of exchange, and drafts are not legal tender. This is why you can refuse to accept a cheque or other debt instrument.
From a historical perspective, it is interesting that one private debt instrument that was accepted as legal tender were National banknotes. The era when National banks existed and their private banknotes were produced was the period from 1864–1935.
But, even in this period, National banknotes were never given the status of full legal tender: at this time, these banknotes were never legal tender for customs payments to the US Treasury, salaries and other debts owed by the US government, or interest payments made on national debt (Patten 1891: 37).
Patten, Claudius B. 1891. The Methods and Machinery of Practical Banking. B. Rhodes & Company, New York.