When a subscriber-
(a) has proceeded on leave preparatory to retirement or if he is employed in a vacation department, on leave preparatory to retirement combined with vacation, or
(b) while on leave, has been permitted to retire or declared by a competent medical authority to be unfit for further service, the amount of subscription and interest thereon standing to his credit in the Fund shall, upon application made by him in that behalf to the Secretary, become payable to the subscriber:
Provided that the subscriber, if he returns to duty, shall, except where the Authority decides otherwise , repay to the Fund for credit to his account, the amount paid to him from the Fund in pursuance of this rule with interest thereon at the rate provided in rule 20 in cash or securities or partly in cash and partly in securities, by installment or otherwise, by recovery from his emoluments or otherwise, as may be directed by the authority competent to sanction an advance for the grant of which, special reasons are required under sub-rule (2) of rule 21.