That is not how an FSA works.
The employee enrolls for a fixed amount per pay period. The total amount for the year is fully available for reimbursement on the first day of the year.
If the OP has valid reimbursements that exceed their contributions to date, the reimbursements must be paid and the employer is prohibited from seeking any repayment from the employee.
This works both ways. If the employee has contributed more than than submitted qualified expenses. Any balance remaining after separation is forfeited.