What I've understand is that the envy stone will be ineffective if the holder does not carry ill intentions or extreme jealousy anymore. So what kang jinha is doing is that he is sacrificing himself so that Seonyun will not be jealous anymore then finally leave siwon alone. Even though in the end, siwon might end up with someone else and not him as the letter had implied.
What I've understand is that the envy stone will be ineffective if the holder does not carry ill intentions or extreme jealousy anymore. So what kang jinha is doing is that he is sacrificing himself so that Seonyun will not be jealous anymore then finally leave siwon alone. Even though in the end, siwon might end up with someone else and not him as the letter had implied.