No City official or employee shall have authority to create an obligation against the City by oral agreement. Purchases shall be made by written purchase order signed by the Purchasing Agent, and agreements for construction work or for personal services shall be by written contract or appointment signed by the City Manager or an administrative officer of the City designated by him. No purchase order or contract shall be valid as an obligation of the City unless it bears a certificate signed by the Director of Finance that the estimated amount thereof has been entered as an encumbrance in the City accounts against an allotment based on a valid appropriation.