Moneys in the Fund not needed for the current operations of the Department of Housing and Urban Development related to insurance under section 1709 of this title shall be deposited with the Treasurer of the United States to the credit of the Fund, or invested in bonds or other obligations of, or in bonds or other obligations guaranteed as to principal and interest by, the United States or any agency of the United States: Provided, That such moneys shall to the maximum extent feasible be invested in such bonds or other obligations the proceeds of which will be used to directly support the residential mortgage market. The Secretary may, with the approval of the Secretary of the Treasury, purchase in the open market debentures issued under the provisions of section 1710 of this title. Such purchases shall be made at a price which will provide an investment yield of not less than the yield obtainable from other investments authorized by this section. Debentures so purchased shall be canceled and not reissued, and the several group accounts to which such debentures have been charged shall be charged with the amounts used in making such purchases.