Russia is the most open Arms dealer in the world. Anyone can buy anything from them, Its there business and most important export industry. They don't mess there Defense Industry with problems of other countries.
They have Sold S400 and su35 to Chinese already. They are selling S400s to Turks as well. Negotiations to sale S400 to Saudis are also underway. They are buying drones and Radars from Israel while Selling S300 systems to Iran and Syria. Is there any other country in the world that have balls enough to sale such advanced system like S300 to Iran and Syria rejecting American pressure altogether at the end?
If we have money we can buy S400 from them too. They don't work on credit only cash. They already supplied Mi35 Gunships to you just recently and ready to sale anything to us provided that we can pay them.
Guess what? They even offered su27 flankers, one of the most advanced aircrafts at that time, in the 90s when your Planners ran after much inferior F16s instead

And India already have a network of S300s that is sufficiently enough to counter every aircraft we have, They don't need S400 for us. This S400 is to counter the rising Threat from Chinese Airforce the PLAAF.
Ignore the Sanghi Lungis who have nothing to do in the world except pitching India against 7 times smaller Pakistan for every and anything