The board of chosen freeholders may, after the execution and approval by the commissioner of a contract for road improvements under this article make without public advertisement, supplemental contracts for additional work found to be necessary as a part of the improvement, provided such contracts shall not cover any additional mileage or length of road, nor alter the general character of the improvement. Before any such contracts shall become binding, and before any work shall be done thereunder, they shall be submitted to and approved by the commissioner in the same manner as provided in this article in case of contracts let after public advertisement.