The refunding bonds may be sold or exchanged for the bonds being refunded, either as a whole or in installments, at any time or times, either at, before, or after the maturity of the bonds being refunded. However, if the refunding bonds are sold, the bonds being refunded pursuant to the provisions of sections 108.400 to 108.410 shall have been outstanding for not less than one year and the bonds shall become due or shall be subject to redemption and payment in accordance with their terms within ten years from the date of issuance of the refunding bonds. If the refunding bonds are sold more than one year prior to the maturity or redemption date of the bonds being refunded, the maturity of the refunding bonds shall not extend beyond the last maturity date of the bonds being refunded, and, upon such sale, the proceeds derived from the sale of the refunding bonds shall be placed in escrow with a bank or trust company having full trust powers and which shall be a member of the Federal Deposit Insurance Corporation. The proceeds shall be invested promptly in direct obligations of the United States of America or of its agencies or instrumentalities or in obligations the principal of and the interest on which are unconditionally guaranteed by the United States of America, which obligations shall mature or be subject to redemption by the holder thereof not later than the respective dates when the proceeds of the obligations, together with the interest accruing thereon, and any other moneys or investments held in escrow, will be required for the purposes intended.