31 U.S.C. § 5153: Counterfeit currency

Disbursing officials of the United States Government and officers of national banks shall stamp or mark the word “counterfeit”, “altered”, or “worthless” on counterfeit notes intended to circulate as currency that are presented to them. An official or officer wrongfully stamping or marking an item of genuine United States currency (including a Federal reserve note or a circulating note of Federal reserve banks and national banks) shall redeem the currency at face value when presented.