As an employer, in some cases, you may over deduct Canada Pension Plan (CPP) contributions. For example, you forget to stop withholding CPP contributions after the maximum amount of pensionable earnings was reached. What should you do? There are two solutions to this problem.
 1. reimburse the employee
If possible, you should reimburse the employee the amount deducted in error and adjust your payroll records to reflect the reduced deduction. This will result in a credit on your CRA payroll account equal to the employee and employer part of the overdeduction. You may then reduce a future remittance in the same calendar year.
Do not include the reimbursed amount on the T4 slip.
2. Apply for a refund
If you cannot reimburse the overpayment, show the total CPP contributions deducted and the correct pensionable earnings on the T4 slip of the employee.
If you reported the employee’s overpayment on the T4 slip, you can ask for refund by completing Form PD24, Application for a refund of Overdeducted CPP Contributions or EI Premiums. Your request must be made no later than four years from the end of the year in which the overpayment occurred.