CRA apparently considers that there is no deduction for repaid employment benefits

CRA considers that only reimbursements of remuneration described in s. 8(1)(n) (such as repayments of income received during a leave when the individual did not return to employment) are deductible, so that if an employee is required in a subsequent year to repay employment benefits received, the employee apparently does not receive a deduction for the repayment.

Neal Armstrong. Summary of 16 March 2015 T.I. 2014-0524371E5 F under s. 8(1)(n).