Whenever advertised bids are received for furnishing materials or commodities for a stated period, and the amount or amounts appropriated and available are not sufficient to cover the requirements for the period stated in the advertisement, said councils, in cities of the first class, may authorize the director of supplies to continue said contract or contracts when further appropriations are made and become available for that purpose, or when further requirements become necessary for the period stated, and this authorization shall be set forth in the said contract or contracts and shall be binding upon the parties affixing their signatures thereto.
53 P.S. § 13373