Before any roads are constructed or improved or bridges are erected or constructed under the provisions of this act, the supervisors of the township of the second class asking for a reward shall furnish to the township commissioner¹ of the State Highway Department² a copy of the plans and specifications of the proposed improvement, together with estimates of the cost of such improvement, unless such plans and estimates have been made by said department. No reward shall be paid under the provisions of this act to any township until the Commonwealth has approved the plans and specifications as filed in the department. When such plans and specifications have been approved, the township commissioner shall certify to the township supervisors the proportion of the cost of such improvement which the Commonwealth agrees to pay. The proportion agreed upon to be paid by the Commonwealth shall not exceed the amount per mile as hereinafter provided in this act, and where the cost of the improvement is less than that set forth in the estimate or contract, a proper deduction shall be made by the Commonwealth for such decrease in cost.