Notwithstanding any other provisions of this chapter or of any other public or general law to the contrary, the retirement board shall permit individuals who purchase service credit to do so in installments. The retirement board shall create, by rule and regulation, provisions allowing that individuals purchasing service credit may do so on an installment plan by payroll deduction not to exceed five (5) years, provided that all purchases must be made prior to retirement. The retirement board shall charge interest at the actuarial assumed rate of return adopted by the board for purchases made on an installment plan.