The principal proceeds received in the sale of any bonds hereafter issued under the authority of this chapter shall be used for the purposes for which the bonds are issued; provided, however, that if for any reason any part of such proceeds shall not be necessary for such purposes then such unexpended part of such proceeds shall be applied to the payment of the principal of or interest on said bonds. All accrued interest and premium received in any such sale shall be applied to the payment of interest or principal on the bonds sold.