The compensation of the Commonwealth's Attorney shall be by salary and such percentage of fines and forfeitures as may be fixed by law, and such salary shall be uniform in so far as the same shall be paid out of the State Treasury, and not to exceed the sum of five hundred dollars per annum; but any county may make additional compensation, to be paid by said county. Should any percentage of fines and forfeitures be allowed by law, it shall not be paid except upon such proportion of fines and forfeitures as have been collected and paid into the State Treasury, and not until so collected and paid.