When a vacancy arises in the Fire Department above the grade of fireman, the Chief of the Fire Department may, with the approval of the City Manager, assign a member of the Department from the next lower rank to fill the vacancy until such time as the absent member shall return to duty or the vacancy be filled by appointment from the eligible list furnished by the Civil Service Board. The member so assigned shall, during his incumbency, receive the salary attached to the grade or position thus temporarily filled.