With the approval of the President, the Secretary of the Treasury may issue retirement and savings bonds of the United States Government and may buy, redeem, and make refunds under section 3111 of this title. The proceeds from the bonds shall be used for expenditures authorized by law. Retirement and savings bonds may be issued only on a discount basis. The maturity period of the bonds shall be at least 10 years from the date of issue but not more than 30 years from the date of issue. The difference between the price paid and the amount received on redeeming a bond is interest under the Internal Revenue Code of 1986 (26 U.S.C. 1 et seq.).