That seems far too simplistic for me. Why target this missile system? It's not like Russia who are the main opponents to this are anywhere close to being USA primary creditors... From what I read this is far more likely to do with a deal between america and russia related to the russian defensive position on Iranian sanctions. At this time Iran will not budge knowing that Russia is backing them all the way. Russia is using Iran as a pawn to ensure that as long as the US retains the policy of placing missiles in Europe then they will defend Iran's enrichment programme; if the USA is claiming that these missiles are required because of the Iranian threat (and not to be used against Russia) then Russia may as well let Iran get nukes. However, now that Obama has dropped the missile plan, russia is now able to switch sides to enable additional sanctions on Iran.
Obama is playing a dangerous game in which he is hoping that by dropping the missile programme, the russians will pressure Iran, Iran will drop their programme (and open their doors), this will pacify Israel who will comply with the settlement requirements; if you look at any Israel commentry they are getting backed into a corner by many nations at this time and will possibly take action against Iran by themselves as they are feeling that the world is against them and they will need to make a decisive move if they are going to protect their own interests. To all intents and purposes at this moment in time it looks like Obama has abandoned them to the rest of the world. So Obama has to act soon or this tinder box will ignite and no-one knows where it will end.
Will Iran respond to Russia? Surely the closer they get to owning a nuke the less they'll care about external pressure.