A municipality may withdraw from the retirement system established by this chapter or may withdraw any of its departments or agencies by the same procedure provided in § 45-21-4 for acceptance of the chapter; provided, that the withdrawal does not relieve the municipality from the liabilities arising from retirement allowances, annuities, or other benefits already granted or determined, or rights and expectatives relative thereto that have become vested in employees of that municipality or in the survivors of those employees, which are discharged by the payment by the retirement system to the employees or beneficiaries concerned, computed according to the actuarial tables in use by the system; and provided, further, that the withdrawal is conditioned upon the written approval of a majority of the employees in the department or agency.