Any contributions by an employee, after the date when his age and service annuity is fixed, shall not increase the amount of such annuity. Such contributions shall be applied toward the extra cost of a minimum annuity where payable over the amount of age and service annuity. The accumulated sum arising therefrom shall be refunded when the employee withdraws from service if he be not entitled to such minimum annuity, or shall be applied toward the extra cost of such minimum annuity if he is eligible therefor, over the age and service annuity to the extent of such extra cost as provided in Section 11-149. The balance, if any, shall be refunded.
     In the event the employee is not entitled to a minimum annuity, or upon death of the employee while in the service after his attainment of age 65 with less than 20 years of service credit at date of death, the accumulated sum arising from employee contributions after his annuity was fixed at age 65 shall be refunded to his widow.