The board of county commissioners shall then fix the amount of the levies necessary to raise the amount of the estimated expenditures as finally determined, less the total of the estimated revenues from sources other than taxation, including such portion of any available surplus as in the discretion of the board it shall be advisable to so use, and such expenditures as are to be met from bond or warrant issues: PROVIDED, That no county shall retain an unbudgeted cash balance in the current expense fund in excess of a sum equal to the proceeds of a one dollar and twenty-five cents per thousand dollars of assessed value levy against the assessed valuation of the county. All taxes shall be levied in specific sums and shall not exceed the amount specified in the preliminary budget.