Each part time employee of the Legislature of Alabama who is eligible and has elected to participate in the State of Alabama Employees’ Retirement System shall be eligible to pay into the State of Alabama Employees’ Retirement System up to one half or 50 percent of the amount of money the part time employee has previously contributed into the fund. Upon paying up to one half or 50 percent of the money the part time employee has previously contributed into the State of Alabama Employees’ Retirement Fund, the legislative employee shall receive a credit proportional to his or her payment of up to one half or 50 percent in addition [additional] retirement credit in years and months from the State of Alabama Employees’ Retirement System. Any employee may elect to pay an amount into the state Employees’ Retirement Fund of less than 50 percent or one half of his or her total part time retirement contribution previously paid and shall then receive a corresponding percentage credit in years and months of service towards retirement.