The District Attorney of the Thirty-eighth Judicial Circuit for Jackson County, Alabama, shall be entitled to receive as an expense allowance, payable from the general fund of such county, a sum equal to thirty percent of the compensation paid such district attorney by the State of Alabama. The expense allowance provided for herein is supplementary to the salary or compensation paid such district attorney by the state, but shall be in lieu of all salaries, allowances, or compensations now payable to such district attorney out of the general fund of such county. The payments provided herein shall be payable in 12 equal monthly installments on warrants properly drawn against the general fund of such county.