The sale of advanced arms to Iran has nothing to do with US-Russia relations and everything to do with Russian-Iranian relations.
Arms sales are right now one of Russia’s biggest generators of foreign currency, as well as keeping Russian arms industry going and workers skills alive. If the price of oil goes back to the $100 barrel level this may change.
Don’t read into my post more than there is.
Russia’s arm sales to a rogue nation like Iran is about profits and their mutual efforts to undermine US power. They also collaborate in other areas that are more dangers to US interests.