In case of actual emergency, and with the consent of the City Manager, any using agency may purchase directly in the open market any supplies, materials, equipment or contractual services whose immediate procurement is essential to prevent delays in the work of the using agency which may vitally affect the life, health or convenience of citizens. The head of such using agency shall send to the Director of Finance a requisition and a copy of the delivery record together with a full written explanation of the circumstances of the emergency, which shall be filed by the Director of Finance as a permanent and public record. The Director of Finance shall by rules and regulations prescribe the procedure under which such emergency purchases may be made.