It is a defence in any proceedings against a person for an offence under section 136 or 137 if the person establishes--
(a) that the offence was committed by the person in the course of the person's employment by another person, or
(b) that on, or in the event of, the sale of the article to which the offence relates, the person received or was to be paid remuneration, by way of commission, from a person other than the owner of the article.