All purchases and contracts made by the city shall be pursuant to a written requisition from the head of the office, department, or agency whose appropriation will be charged, and shall be first submitted to and approved by the city manager. Upon approval by the city manager, the same shall be delivered to the city treasurer or his designee for encumbrance. Absent the approval of the city manager and city treasurer, or their respective designees, the same shall not constitute a valid claim against the city of Purcell.