Is it permissible for an employee to take a commission from a customer he brought to the company without its knowledge?

As long as attracting customers is not part of your job description and you have no functional relation to it, then the default ruling is that it is permissible to take a commission for it. Imam Malik was asked about the broker's fee, and he said: "There is no harm in that." Ibn Sirin, Ata, Ibrahim, and Al-Hasan held the view that a broker's fee is permissible. So, if you bring a customer to the company and agree on a commission, this is permissible. The commercial intermediary (broker) may take commission from either party or from both parties if agreed upon. Otherwise, the custom is that it is taken from the one who requested your mediation. It states in "Fatawa of the Permanent Committee": "It is permissible for the dalal - broker - to take a known percentage of the price at which the commodity settles, in return for brokering it, and he collects it from the seller or the buyer, according to agreement, without injustice or harm."