Is the wage I deduct from the price of parts I buy from the factory permissible or forbidden?

If the employer and his partner permit you to do drawings outside your working hours for a fee, this is permissible, provided the fee is known. This is because an unknown fee invalidates the hiring agreement, in which case the employee is entitled to a fair wage (ajr al-mithl). The questioner may receive the fair wage in cash or by deducting it from the price of an item he purchases from the company. However, this is only permissible after consulting the financial manager, and not based solely on the word of one of the partners.