The head of a department may, in his or her discretion, grant any civil service employee in his or her department a leave of absence without pay for a period not exceeding fifteen (15) calendar days. Absence for a longer period of time not to exceed one year may be granted by permission on written request of the employee, giving the reasons therefor, approved by the head of the department and the mayor. Unless the commission provides otherwise for any particular case, a leave of absence for more than three months shall not entitle the employee when returning to duty to resume his former position, but such employee shall be entitled only to standing on the preferred eligible list for said position; this limitation shall not apply to absence on account of disability caused by sickness or injury, or to leave of absence during which the employee performs temporary service in a different position in the same department, or temporary service in any position in any other department of the city government.