Is it permissible for an employee to exchange inner garments (undershirts) purchased at a discount for a different size that was unavailable at the time of the discount, when the desired size becomes available later?

The employee is an agent for the employer, and he is not permitted to act except within the scope of what he has been authorized to do. Accordingly, the responsible employee is not permitted to exchange goods designated for discount or to transfer the discount to another size for which he has not been authorized.