What is the ruling on commission for facilitating a commercial transaction if the mediator is a purchasing officer for the buyer and desires a portion of the commission?

Intermediation between a seller and a buyer is called brokerage (simsarah) and it is permissible. There is no harm in taking a fee for it. Therefore, there is no objection for you to act as an intermediary and take a known commission for it, provided that the intermediated item is permissible شرعاً (according to Islamic law). As for the purchasing agent who was authorized by the buyer to make the purchase and who asks you for a part of this commission, it is not permissible for him to take anything from it for himself unless the principal (the one who authorized him) grants him permission.