The compensation of county commissioners of any county having a population of not less than one hundred thousand (100,000) nor more than six hundred thousand (600,000), according to the United States census of 1970, shall be not less than twenty-five thousand dollars ($25,000) a year, but any such county may by private act provide for annual compensation in a larger amount. The salary fixed by this section shall control over any prior private or public enactment providing for an automatic salary adjustment for any county commissioner and any such provision is hereby declared to be superseded by this section.