Section 112.

When it becomes necessary, in the opinion of the Director or Head of the appropriate Department, in the prosecution of any work, or improvement under contract, to make alterations or modifications, such changes shall be made only upon the order of such Director or Head in writing and upon the approval of the Board of Control. No such order shall be effective until the price to be paid for the work or material, or both, under the altered or modified contract, shall have been agreed upon in writing and signed by the Contractor and the Director or Head of the Department in behalf of the City.