Is it permissible to request a commission from the seller in exchange for purchasing goods for the company, with the possibility of increasing the price of the merchandise, knowing that this work is outside the scope of one's primary job duties and is unpaid?

You are not permitted to take a commission from the seller as an agent of the company, even if you did not request it, unless you have the company's permission. The gift resulting from the sale is considered an increase in the price and belongs to the owner of the garment. The fact that the work is outside your official duties does not permit you to take any right belonging to the company without its knowledge. You can refuse to perform the task unless there is compensation, but if you agree to it without a commission condition, then you are not entitled to take anything that belongs to the company.