The Export-Import Bank of the United States is authorized to issue from time to time for purchase by the Secretary of the Treasury its notes, debentures, bonds, or other obligations; but the aggregate amount of such obligations outstanding at any one time shall not exceed $6,000,000,000. Such obligations shall be redeemable at the option of the bank before maturity in such manner as may be stipulated in such obligations and shall have such maturity as may be determined by the Board of Directors of the bank with the approval of the Secretary of the Treasury. Each such Bank obligation issued to the Treasury after January 4, 1975, shall bear interest at a rate not less than the current average yield on outstanding marketable obligations of the United States of comparable maturity during the month preceding the issuance of the obligation of the Bank as determined by the Secretary of the Treasury. The Secretary of the Treasury is authorized and directed to purchase any obligations of the Bank issued hereunder and for such purpose the Secretary of the Treasury is authorized to use as a public-debt transaction the proceeds of any securities issued after July 31, 1945, under chapter 31 of title 31, and the purposes for which securities may be issued under that chapter are extended to include such purpose. Payment under this section of the purchase price of such obligations of the Bank and repayments thereof by the Bank shall be treated as public-debt transactions of the United States.