Any unexpended balance of such security proceeds remaining after the completion of the acquisition or improvement and equipment of the project or the completion of the purpose or purposes for which such securities were issued shall be paid immediately into the fund created for the payment of the principal of said securities 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.