A member who has met the age and service requirements to receive a retirement allowance from this retirement system as provided in section 81 may withdraw all of the accumulated contributions, if any, if the member is entitled to receive credit for all or part of the service in another retirement system. Withdrawal shall result in the forfeiture of prior service and contributory plan membership service credited to the member upon payment of the refund. A member who has met the age and service requirements to receive a monthly allowance under the noncontributory plan, or combination of contributory and noncontributory plan service, by written waiver and withdrawal of the accumulated contributions, if any, may forfeit a monthly allowance if the member is entitled to receive credit for all or part of his or her service in another retirement system. Once a retirant is in receipt of a retirement allowance from this retirement system, it shall not be forfeited.