(a) Beginning on the effective date of this amendatory Act of 1991, an employee who is still in service and is eligible to establish optional service credit under this Article for any period during which he was not an active participant in the Fund need not establish credit for the entire period for which he is eligible, but may instead elect to establish credit for only a fraction of that period. In such cases, the credit established shall be deemed to relate to the earliest period for which that type of credit may be established. However, in no event shall any such credit be granted until the employee contributions required for that credit, if any, have been paid.
     (b) Notwithstanding Section 11-163 or any other provision of this Article, beginning on the effective date of this amendatory Act of 1991, an employee who has returned to service and is required (or authorized) to restore service credit that was surrendered upon payment of a refund need not restore such credit in full, but may instead elect to restore only a fraction of the surrendered service credit, or none of it. If only some of the surrendered credit is to be restored, the credit shall be restored in the order in which it was earned, and the board shall determine the amount that must be repaid by the employee to the Fund in order to restore the credit, based on the corresponding fraction of the refund, plus interest as required by the other provisions of this Article. In no event shall any such credit be restored until the payment required for that credit has been paid, and in no event shall any benefit be granted based on surrendered credit that has not been restored.