The Secretary may prescribe the conditions and frequency for receiving payment under obligations of a government of a foreign country acquired under the laws referred to in subsection (a) of this section. A sale of an obligation acquired under those Acts shall at least equal the purchase price and accrued interest. The proceeds of obligations sold under this section and payments received from governments on the principal of their obligations shall be used to redeem or buy (for not more than par value and accrued interest) bonds of the United States Government issued under this chapter. If those bonds cannot be redeemed or bought, the Secretary shall redeem or buy other outstanding interest-bearing obligations of the Government that are subject to redemption or which can be bought at not more than par value and accrued interest.