The Council shall have power to do any public work or make any public improvement by the employment of the necessary labor and the purchase of the necessary supplies and materials, with separate accounting as to each improvement so made, or to do such work by contract duly let after competitive bidding. Where competitive bids are secured, the City, or any City department qualified to do the work, may, at the direction of the Council, enter a bid on an equal footing with other bidders. The Council, in its discretion, shall have the power to reject any or all bids. On all jobs where the plans thereof are required by law to be prepared by a professional engineer, or the total cost thereof exceeds $2000. 00, each contractor shall be required to file a performance bond, to be approved by the City Attorney, covering labor and material and for the proper completion of the work. Copies of all contracts shall be filed in the office of the Clerk. The Council shall also have power to do any public work or make any public improvement under any legally constituted plan under which the labor is furnished by any other governmental unit, department, or agency of the United States or the State of Michigan, or which is wholly or in part financed by them or either of them.