Any unexpended balance of the proceeds of such securities remaining after the completion of the acquisition or improvement of properties pertaining to the project or otherwise the completion of the purpose or purposes for which such securities were issued shall be credited immediately to the fund or account created for the payment of the interest on or the principal of the securities, or both principal and interest, and shall be used therefor, subject to the provisions as to the times and methods for their payment as stated in the securities and the proceedings authorizing or otherwise appertaining to their issuance, or so paid into a reserve therefor, or any combination thereof, as the commission may determine.