§ 18. Change in contracts. Any change, addition or deduction made in any contract by reduction in price, by addition in price or otherwise must be done by an order on contract which may be issued by the commissioner of general services. This order shall specifically set forth conditions of the change or other modification and shall be approved by the board, department, commission or officer having jurisdiction. After having been approved by him the order shall be forthwith filed in the office of the state comptroller.