Is it permissible for a specialist physician to pay a commission to general practitioners in exchange for referring patients to him, whether this commission is charged to the patient or deducted from the physician's net income?

It is permissible for your colleagues to guide patients to your clinic, provided that there is no deception involved, such as them knowing that someone else is better than you and directing patients to you for personal gain. This is supported by the Prophet's saying, peace and blessings be upon him: "Religion is an advice." So, if the guidance is free from deception and the commission paid is clearly defined, there is no objection to it, and it is considered a permissible form of brokerage.