The city councils of any city shall have power to establish an ordinance creating a purchasing department for the city. The administrative head of such department shall be called the city purchasing agent and he shall be appointed by the mayor with the approval of the city council, or the city council by ordinance may assign the duties of the city purchasing agent to an existing municipal officer. The city purchasing agent shall make all purchases of materials, equipment, and supplies for all departments, officers and agencies of the city under such rules and regulations as he shall promulgate subject to the approval of the city councils. In any city adopting the provisions hereof all departments, officers and agencies of the city shall requisition to the purchasing agent for all purchases in accordance with rules and regulations of said purchasing agent and the city treasurer shall not charge purchases against city funds unless and until the purchase order has been approved by the purchasing agent, subject to such exceptions in case of emergency as the councils may approve in accordance with such rules and regulations. The rules and regulations relative to the purchasing department shall include a requirement that all purchases of an amount over $100 dollars shall be made by competitive bidding subject to such exceptions as the councils may approve. In any city adopting the provisions hereof, any special provisions of the charter of said city inconsistent herewith relative to departments making their own purchases are hereby repealed to the extent of such inconsistency.