The proceeds from the sale of any bonds issued under authority of this chapter shall be applied only for the purpose for which the bonds were issued; provided, however, that any premium and accrued interest received in any such sale shall be applied to the payment of the principal of or the interest on the bonds sold; and provided further, that if for the purpose for which the bonds were issued, such unneeded portion of the proceeds shall be applied to the payment of the principal of or the interest on the bonds.