Is it permissible for an employee to receive a commission in his capacity as an agent for a company that sells computers, with the purchasing company's knowledge, given that the price of the devices will not change whether through his mediation or not?

It is permissible to act as an intermediary between a seller and a buyer, which is known as brokering (simsarah), and it is permissible to take a fee for it. Your work as an agent for a computer company, from which you receive a commission for your sales, falls under this category. It is permissible for a broker to take a commission from both parties, the seller and the buyer. Your position at the purchasing company does not affect your entitlement to a commission from the other company, unless you violate the terms of your contract with either party.