The legislature may from time to time, by general or local laws, fix, alter and regulate the costs and charges of court in Shelby county and the fees, commissions, percentages, allowances, and compensation to be charged or received by the judge of probate, sheriff, tax assessor, tax collector, clerk of the circuit court, register in chancery, and any other officer of Shelby county, including the right to place any of such officers on a salary, provide for the operation of their respective offices on such basis, and provide that any and all fees, commissions, percentages, or allowances charged or collected by them, including all fees, allowances, commissions, and percentages of the tax assessor and tax collector for assessing and collecting municipal taxes, shall be paid into the county treasury; provided, that no law shall be effective to change the method of compensating any officer, nor to diminish his compensation, during the term for which he shall have been elected or appointed, and no law changing the method of compensating county officers shall be effective unless it shall have been approved by a majority of the qualified electors of the county who vote thereon at a referendum election held for such purpose.