What is the ruling on an employee working for his own company during his free time for an additional fee, while formally dealing with an external company as an intermediary?

If you work for another company after official working hours and it sells its product to your company, there is no harm, provided that you are not the one responsible for purchasing for your company. This is because an agent cannot buy from himself unless authorized by his principal. The majority of jurists have held that it is not permissible for a selling agent to sell to himself unless the principal grants permission. Therefore, if you are responsible for purchasing the product for your company, it is not permissible unless your company is aware of it. However, if the owner or another person is the one purchasing the product, then there is no harm in what you are doing.