Refunding obligations issued as provided in section 67-6412 may be sold or exchanged for outstanding obligations issued under this act and, if sold, the proceeds thereof may be applied, in addition to any other authorized purposes, to the purchase, redemption or payment of such outstanding obligations. Pending the application of the proceeds of any such refunding obligations, with any other available funds, to the payment of the principal, accrued interest and any redemption premium on the obligations being refunded, and, if so provided or permitted in the resolution authorizing the issuance of such refunding obligations or in the trust agreement securing the same, to the payment of any interest on such refunding obligations and any expenses in connection with such refunding, such proceeds may be invested in direct obligations of, or obligations the principal of and the interest on which are unconditionally guaranteed by the United States of America which shall mature or which shall be subject to redemption by the holders thereof, at the option of such holders, not later than the respective dates when the proceeds, together with the interest accruing thereon, will be required for the purposes intended.