The Mayor may, subject to any regulations which the Common Council may prescribe and within the amounts of the budget appropriations, make purchases and enter into contracts on behalf of the city. Every such contract and purchase exceeding an amount to be established by the Common Council shall require the prior approval of the Common Council. The Mayor may transfer to or between the departments, offices and agencies, or sell surplus or obsolete supplies, materials, and equipment, subject to such regulations as the Common Council may prescribe. The city shall have the power to require all bidders to post bonds to secure the performance of any contract and all claims for labor and material used in the work. The city shall have the power to reject any supplies, as well as any other public work, and buy supplies on the open market at a price less than the lowest bid received, or if no bids are received, it may direct the purchase of supplies in the open market.