Plans providing for retirement benefits established under this chapter shall provide that mandatory contributions made by a participating employee shall be returned to such employee or his estate in the event of death before retirement. If the employee is separated from employment prior to the time he is eligible for retirement benefits, such contributions shall be returned unless the employee chooses to claim his vested benefits, in which case the employee contributions shall remain with the fund until such time as the employee becomes eligible for the vested benefits. Such contributions may be returned without interest or with such interest as is provided in the plan.