Except as provided in subsection (b) of this section, a member may not purchase service credit under this title if the member is separated from employment. A member who is separated from employment may purchase service credit under this title for a leave of absence approved by the Board of Trustees under regulations that apply to all members, if the member purchases the service credit within 60 days after the expiration of the leave of absence. For good cause shown, the Executive Director, in the Executive Director’s sole discretion, may extend the time period to purchase service credit under paragraph (1) of this subsection if: the purchased service credit would allow the member to meet the eligibility service requirements to apply for an ordinary disability retirement under § 29–105(a) of this article; and the member completes the purchase of the service credit before the earlier of: 4 years after the expiration of the leave of absence; or the date membership ends. The Board of Trustees shall adopt regulations to carry out this subsection.