The expenditure of money by a county shall be in accordance with the annual appropriations of the board of county commissioners. All bills in connection with any public office or undertaking shall be approved by the official or officials in charge of the office or undertaking before being allowed by the board of county commissioners. At the closing of the auditor’s books on December thirty-first, the balance to the credit of each annual appropriation shall become a part of the general unappropriated balance in the county treasury. A special appropriation, however, shall not lapse until the work for which it was made has been completed, the bills paid, and the account closed, provided that the county fair fund mentioned in section 11-39-02 shall not be affected by this section but shall be maintained as a continuing fund for the maintenance of the county fair.