Russia doesn't deliver the S-300 to Iran because it's much less effective that announced. They deliver it to countries that won't use it e.g. Greece .If Israel or the U.S. prove that this system is inefficient, Russia can forget the sale of such type of armament. Do you remember what happens in Syria with Russian air defense ?
Yemen's Houthi rebels commit to peace plan in letter to UN chief, BBC reports (Haaretz)
from the article: Russia says in talks with Iran on new nuclear plants