What is the ruling on taking a 20% commission from a contractor for mediating the installation of doors for another company, with the knowledge and consent of the latter company, and provided that this does not affect the original company's selling prices? Is there any suspicion of impropriety or unlawfulness in this?

There is no objection to receiving payment for installing doors, if the company you work for explicitly permits it, or if common practice does not prohibit it. This is known as "brokerage" or "commission." Jurists such as Ibn Sirin, Ata, Ibrahim, and Al-Hasan permitted it, citing the hadith: "Muslims are bound by their conditions."