In the case of an emergency, any department head, with the approval of the Mayor, may purchase directly any supplies, materials or equipment not costing in excess of one thousand dollars ($1,000) when immediate procurement is necessary to the continuation of the work of the department. Such purchases and the emergency causing them shall be reported in detail to the Purchasing Agent within a week from the time such purchases are made, and such reports shall be filed by the Purchasing Agent as permanent public records.