40 ILCS 5/14-104.3
Notwithstanding provisions contained in Section 14-103.10, any member who at the time of retirement and after December 6, 1983 receives compensation in a lump sum for accumulated vacation, sickness, or personal business may receive service credit for such periods by making contributions within 90 days of withdrawal, based on the rate of compensation in effect immediately prior to retirement and the contribution rate then in effect. Exercising the option provided in this Section shall not change a member's date of withdrawal or final average compensation for purposes of computing the amount or effective date of a retirement annuity. Any annuitant who establishes service credit as herein provided shall have his retirement annuity adjusted retroactively to the date of retirement.
40 ILCS 5/14-104.3