As approved by the board of directors, Federal Prison Industries, to such extent and in such amounts as are provided in appropriations Acts, is authorized to issue its obligations to the Secretary of the Treasury, and the Secretary of the Treasury, in the Secretary’s discretion, may purchase or agree to purchase any such obligations, except that the aggregate amount of obligations issued by Federal Prison Industries under this paragraph that are outstanding at any time may not exceed 25 percent of the net worth of the corporation. For purchases of such obligations by the Secretary of the Treasury, the Secretary is authorized to use as a public debt transaction the proceeds of the sale of any securities issued under chapter 31 of title 31 after the date of the enactment of this section, and the purposes for which securities may be issued under that chapter are extended to include such purchases. Each purchase of obligations by the Secretary of the Treasury under this subsection shall be upon such terms and conditions as to yield a return at a rate not less than a rate determined by the Secretary of the Treasury, taking into consideration the current average yield on outstanding marketable obligations of the United States of comparable maturity. For purposes of the first sentence of this paragraph, the net worth of Federal Prison Industries is the amount by which its assets (including capital) exceed its liabilities.