The board may issue and sell the bonds of the district, when authorized, at not less than par value, and the proceeds of the sale shall be placed in the county treasury or other depository for the district’s fund to the credit of the proper district fund and shall be applied exclusively to the purposes and objects mentioned in the ordinance or resolution ordering the holding of the bond election; provided, however, that when said purposes and objects have been accomplished any moneys remaining in said fund shall be transferred to the general fund of the district and may be used by the board for any lawful purpose of the district.