Shaykh Saalih al-Fawzaan hafidhahullaah was asked:
Q: Is it allowed to donate one of my kidneys to my Muslim brother that suffers from kidney failure?
A: This is the ruling that The Committee of Senior Scholars released; that it is permissible to do so upon two conditions:
(1) That it does not affect the donor, and there is no danger in the donor doing so, and it does not affect him in his health.
(2) And that the recipient was in a dire need for such.
With (these) two conditions. So if the two conditions are met then the donation is permissible. Yes.