The warm relations that exist between Russia and Iran should not mislead you when it comes to money. In view of what appears to be an impasse in the nuclear talks with the US, Iran has plans for how to still maintain its share of the world oil market.
Due to US sanctions, tens of millions of barrels of Iranian oil are "stuck" without a buyer on ships in Singapore and elsewhere in Asia.
Iran intends to offer China to purchase a barrel of oil at a price 5 - 7 dollars lower than what Russia is offering to China.
The discounted oil is the only way for Iran to maintain its position in the market as long as the sanctions are not removed. Iran sells its oil to China indirectly since the Chinese do not want to "get into trouble" with the Americans by violating the sanctions.