31 CFR 100.3: Lawfully held coin and currencies in general.

The official agencies of the Department of the Treasury will continue to exchange lawfully held coins and currencies of the United States, dollar for dollar, for other coins and currencies which may be lawfully acquired and are legal tender for public and private debts. Paper currency of the United States which has been falsely altered and coins altered to render them for use as other denominations will not be redeemed since such currency and coins are subject to forfeiture under Title 18, United States Code, section 492. Persons receiving such currency and coins should notify immediately the nearest local office of the U.S. Secret Service of the Department of the Treasury, and hold the same pending advice from the Service.