Before any order or contract for improvement, service, supplies, etc., is issued by the city, council, or any board, officer, or employee, it shall be presented to the city controller for his approval. The controller shall examine such order or contract and shall not approve it unless he finds that an appropriation has been made which has not been exhausted, and that the order or contract has been otherwise legally authorized and approved. All orders and contracts which have not been approved by the controller shall be void.