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.