The Treasurer may invest temporarily in direct obligations of the United States of America such portion of the proceeds of the sale of regional marketing bonds as he deems to be available for such purpose. If the proceeds of any bond issue exceed the purchase price or cost of construction of facilities for which such bonds are issued, the surplus shall be expended in payment of principal of and interest on bonds issued under this chapter when the same become due or in the purchase at the market price of any outstanding bonds, provided such price shall not exceed the price at which such bonds are in the same year redeemable. All bonds redeemed or purchased shall forthwith be cancelled and shall not again be issued.