Upon the written request of a member who ceases to be an employee, a lump sum amount shall be paid to such person equal to the total amount credited to such member's account at the time the member ceases to be an employee. If such member dies before payment has been made, such payment shall be made to such person as the member has nominated, by written designation filed with the board; otherwise to the member's estate. If no such request is made, the member's account shall continue to accrue earnings in the same manner as any member's account; provided, however, that if any member who ceases to be an employee has less than $3,500.00 credited to such member's account, the board may, at its option, require such member to withdraw all such moneys and the member's account shall be closed.