The several counties of this Commonwealth shall have the power and they are hereby authorized, by resolution of the county commissioners or a majority of them, to reimburse and pay to contractors, in addition to the contract price, on account of the improvement of any highways, within the respective limits of said counties, under contracts between said counties and such contractors approved by the State Highway Department prior to the order of the Interstate Commerce Commission of the United States of America, increasing railroad freight rates, made the twenty-ninth day of July, one thousand nine hundred and twenty, the additional or excess amount of freight charges necessary to the performance of such contracts occasioned by said increase of freight rates.