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 purchase or contract does not exceed $1,500.00. All other purchases shall be made and all other contracts let by the council after the recommendation of the city manager has first been obtained. 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.