; and (2) the purposes for which securities may be issued under that chapter are extended to include any purchase under this subsection. (g) Maximum outstanding holding The Secretary of the Treasury shall purchase instruments issued under subsection (e) to the extent that the purchase would not increase the aggregate principal amount of the outstanding holdings of obligations under subsection (e) by the Secretary of the Treasury to an amount that is greater than $2,000,000,000. (h) Rate of return Each purchase of obligations by the Secretary of the Treasury under this section shall be on terms and conditions established to yield a rate of return determined by the Secretary of the Treasury to be appropriate, taking into account the current average rate on outstanding marketable obligations of the United States as of the last day of the month preceding the purchase. (i) Sale of obligations The Secretary of the Treasury may at any time sell, on terms and conditions and at prices determined by the Secretary of the Treasury, any of the obligations acquired by the Secretary of the Treasury under this section. (j) Public debt transactions All redemptions, purchases, and sales by the Secretary of the Treasury of obligations under this section shall be treated as public debt transactions of the United States. 107. General provisions (a) Immunity from impairment, limitation, or restriction (1) In general All rights and remedies of the Administration (including any rights and remedies of the Administration on, under, or with respect to any mortgage or any obligation secured by a mortgage) shall be immune from impairment, limitation, or restriction by or under— (A) any law (other than a law enacted by Congress expressly in limitation of this paragraph) that becomes effective after the acquisition by the Administration of the subject or property on, under, or with respect to which the right or remedy arises or exists or would so arise or exist in the absence of the law