Purchasing and contracting procedures, including bidding procedure when it will serve the best interest of the City, shall be established by ordinance. The City Manager shall be responsible for purchasing and a system of centralized purchasing shall be developed and maintained. The authority to contract in behalf of the City is hereby vested in the City Commission, provided, that the City Manager shall have power to bind the City as to routine or operational items. No contract involving the expenditure of more than five thousand dollars ($5,000), unless it be routine or operational, shall be authorized or executed until after the Comptroller has first certified that an appropriation has been made therefor or that sufficient funds will be available to finance the same. For this purpose the Comptroller may rely upon City Commission commitments to provide therefor in ensuing budgets or by issuing bonds or other evidences of indebtedness for the same. No contract shall be made with any person, firm or corporation in default to the City.