Refunding obligations issued as provided in § 42-55-15 may be sold or exchanged for outstanding obligations issued under this chapter and, if sold, the proceeds may be applied, in addition to any other authorized purposes, to the purchase, redemption, or payment of outstanding obligations. Pending the application of the proceeds of any 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 provided or permitted in the resolution authorizing the issuance of refunding obligations or in the trust agreement securing the refunding obligations, to the payment of any interest on the refunding obligations and any expenses in connection with the refunding, the 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 which shall mature or which shall be subject to redemption by their holders, at the option of those holders, not later than the dates when the proceeds, together with the interest accruing on the proceeds, will be required for the purposes intended.