53 Pa. Stat. § 12672
Every contract for public improvements shall be based upon an estimate of the cost of such improvement, or the part thereof to be done under such contract, furnished by the proper officers through the department having charge of the improvement, and no bid in excess of such estimate shall be accepted. Such estimate and contract may be limited to any unit or units, part or parts, of the improvement, and contracts may thereupon be made, and the work on such unit or units, part or parts, proceed to the extent of the funds available for the purposes thereof. Every such contract shall contain a clause that it is subject to the provisions of this act, and the liability of the city thereon shall be limited by the amounts which shall have been or may be from time to time appropriated for the same.
53 P.S. § 12672