(a) Before retirement. An employee who withdraws and elects not to receive a refund and later returns to service shall receive credit for the service previously rendered for which contributions were made and remained in the Fund.
     (b) After retirement. When any person receiving a retirement annuity re-enters service, payments of an annuity to that person shall be suspended while such person remains in service. When that person again withdraws, payments of the annuity previously granted shall be resumed and shall be adjusted to reflect the annual increases under Section 13-302(d) of this Article during the period of suspension. The surviving spouse‘s annuity shall remain fixed at the amount set at the first retirement date, subject to adjustments for annual increases as provided in Section 13-306(b) of this Article. No contributions shall be made by the formerly retired employee during service after re-entry, nor shall the employee be entitled to credit for service during such reemployment.