31 USC 5119 - Redemption and cancellation of currency

(a) Except to the extent authorized in regulations the Secretary of the Treasury prescribes with the approval of the President, the Secretary may not redeem United States currency (including Federal reserve notes and circulating notes of Federal reserve banks and national banks) in gold. However, the Secretary shall redeem gold certificates owned by the Federal reserve banks at times and in amounts the Secretary decides are necessary to maintain the equal purchasing power of each kind of United States currency. When redemption in gold is authorized, the redemption may be made only in gold bullion bearing the stamp of a United States mint or assay office in an amount equal at the time of redemption to the currency presented for redemption.
(b) 
(1) Except as provided in subsection (c)(1) of this section, the following are public debts bearing no interest:
(A) gold certificates issued before January 30, 1934.
(B) silver certificates.
(C) notes issued under the Act of July 14, 1890 (ch. 708, 26 Stat. 289).
(D) Federal Reserve notes for which payment was made under section 4 of the Old Series Currency Adjustment Act.
(E) United States currency notes, including those issued under section 1 of the Act of February 25, 1862 (ch. 33, 12 Stat. 345), the Act of July 11, 1862 (ch. 142, 12 Stat. 532), the resolution of January 17, 1863 (P.R. 9; 12 Stat. 822), section 2 of the Act of March 3, 1863 (ch. 73, 12 Stat. 710), or section 5115 of this title.
(2) Redemption, cancellation, and destruction of currency.— 
The Secretary shall
(A) redeem any currency described in paragraph (1) from the general fund of the Treasury upon presentment to the Secretary; and
(B) cancel and destroy such currency upon redemption.

The Secretary shall not be required to reissue United States currency notes upon redemption.

(c) 
(1) The Secretary may determine the amount of the following United States currency that will not be presented for redemption because the currency has been destroyed or irretrievably lost:
(A) circulating notes of Federal reserve banks and national banks issued before July 1, 1929, for which the United States Government has assumed liability.
(B) outstanding currency referred to in subsection (b)(1) of this section.
(2) When the Secretary makes a determination under this subsection, the Secretary shall reduce the amount of that currency outstanding by the amount the Secretary determines will not be redeemed and credit the appropriate receipt account.
(d) To provide a historical collection of United States currency, the Secretary may withhold from cancellation and destruction and transfer to a special account one piece of each design, issue, or series of each denomination of each kind of currency (including circulating notes of Federal reserve banks and national banks) after redemption. The Secretary may make appropriate entries in Treasury accounts because of the transfers.