The president and associate members of the board shall each receive an annual salary in the sum of four thousand two hundred dollars ($4,200), payable in equal monthly installments of three hundred fifty dollars ($350) by warrant issued by the board and signed by the president. Such salary shall be paid from the general fund or from the tax levied by the state on gasoline and other motor fuels and oils or by apportionment thereto as the board from time to time may determine. If a general law is now or becomes effective whereby the salaries of the county governing bodies of Alabama are fixed at a higher level than those specifically fixed herein, then such salaries prescribed herein shall automatically increase to the maximum level prescribed under such general law for counties within the population range of Covington County, that is, the president of the board shall receive a salary corresponding in amount to that fixed under the general law for the presiding officer of the court of county commissioners and each associate member of the board shall receive an amount equal to that fixed under general law for each associate member of the court of county commissioners. If any discretion in the fixation of salaries is given by general law to the court of county commissioners in the fixation of its own salaries, then the Board of Revenue of Covington County shall have like authority to increase but not to lower the salaries herein specifically fixed.