Section 4-3   Purchases and sales.
   (a)   The City Manager shall contract for and purchase or issue purchase authorizations for, all supplies, materials, and equipment for the offices, departments, and agencies of the City Government or sell surplus or obsolete supplies, materials, and equipment, subject to such regulations as the City Council may prescribe.
   (b)   Before the purchase of, or contract for, any supplies, materials, or equipment, or sales of any surplus or obsolete supplies, materials, or equipment, ample opportunity for competitive bidding, under such regulations and with such general exceptions, as the City Council may prescribe, shall be given; but the City Council shall not exempt an individual contract, purchase or sale from the requirement of competitive bidding.