The Secretary of the Treasury is authorized in the Secretary’s discretion to purchase any obligations issued pursuant to subsection (b) of this section, as now or hereafter in force, and for such purpose the Secretary of the Treasury is authorized to use as a public debt transaction the proceeds of the sale of any securities hereafter issued under chapter
31 of title
31, and the purposes for which securities may be issued under chapter
31 of title
31 are extended to include such purchases. The Secretary of the Treasury shall not at any time purchase any obligations under this subsection if such purchase would increase the aggregate principal amount of the Secretary’s then outstanding holdings of such obligations under this subsection to an amount greater than $2,250,000,000. 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 determined by the Secretary of the Treasury, taking into consideration the current average rate on outstanding marketable obligations of the United States as of the last day of the month preceding the making of such purchase. The Secretary of the Treasury may, at any time, sell, upon such terms and conditions and at such price or prices as the Secretary shall determine, any of the obligations acquired by the Secretary under this subsection. All redemptions, purchases, and sales by the Secretary of the Treasury of such obligations under this subsection shall be treated as public debt transactions of the United States.