In addition to any other expense allowances provided by law for the members of the county commission of Coffee County, Alabama, each member of such commission is entitled to and shall receive a monthly expense allowance not to exceed four hundred fifty dollars ($450). The exact amount of such additional allowance shall be determined, from time to time, by subtracting from four hundred fifty dollars ($450) the amount of any new additional monthly compensation, whether in the nature of salary, salary supplement, or expense allowance, that may be provided by general law for such members. Such additional expense allowance shall be paid to such members in the usual manner from the county general fund.