The county commissioners may receive subscriptions and donations in money and real or personal property which shall be applied in the construction or the improvement of said road. Any aid or donation by the state or the United States, or by cities, or from any other source shall be placed in a special fund for such improvements and shall be applied by the board of county commissioners before apportioning the remainder of the cost between the county, township and benefit district. Such state and federal aid and donations and special assessments paid in full shall be used in paying off the warrants and interest accumulating thereon which were issued during the progress of the improvement.