Each district judge in the Fifteenth Judicial Circuit, in addition to the salary paid by the state, shall receive a supplemental salary from the general fund of the county composing the judicial circuit. The supplemental salary shall at all times equal 90 percent of the total supplementary salary paid by the county to each circuit judge and shall be paid in equal installments at the same times and in the same manner that the salaries of the county employees are paid. The Montgomery County Commission shall pay the supplemental salary to each district judge out of the funds of the county or any other funds available for this purpose. The salary is in lieu of any other supplemental salary paid by the county and represents the total supplemental salary to be provided by the county for the district judges.