No purchase shall be made or obligation incurred except upon authorization of the board, or authorization by general ordinance, and no expense shall be incurred, except for a purpose for which no prior appropriation is required by law, if the amount thereof is not covered by an unexpended appropriation for the purpose. The treasurer shall keep current accounts of the amounts appropriated and the amounts spent for each appropriation, showing the unexpended appropriations at all times. (1965 Code §11.04)