The city manager shall be the chief purchasing agent of the city. All city purchases and contracts shall be made or let by the city manager when the amount of the purchases or contract does not exceed $5,000.00. All other purchases shall be made and all other contracts let by the city manager only after prior approval of the Council thereof. All contracts, bonds, and instruments of any kind to which the city is a party shall be signed by the mayor and the city manager on behalf of the city and shall be executed in the name of the city.