The City Manager, subject to any regulations which the Council may prescribe, shall contract for and purchase, or issue purchase orders for, all supplies, materials, and equipment for the offices, departments, and agencies of the City government. Every such contract or purchase exceeding an amount to be established by ordinance shall require the prior approval of the Council. The City Manager also may transfer to or between offices, departments, and agencies, or sell surplus or obsolete supplies, materials, and equipment subject to such regulations as the Council may prescribe. Before the purchase of or contract for any supplies, materials, or equipment, or the sale of any surplus or obsolete supplies, materials, or equipment, ample opportunity for competitive bidding under such regulations and with such exceptions as the Council may prescribe shall be given; but the Council shall not except a particular contract, purchase, or sale from the requirement of competitive bidding.