Iran, Russia, and the Bushehr Reactor

Russia says that Iran is failing to 'pay up' on funds due for completion of the Bushehr reactor. Iran says that Russia's Atomstroiexport has financial problems of its own, and has offered to pay additional money to get the deal moving again.

Other have wondered what has gone wrong in Teheran that has allowed payments to fall behind, but I've seen no reason to conclude that Russia is telling the truth and Iran is lying. So which is more likely - that a Russian company has financial trouble and demands more - knowing that Iran has no one else to go to - or that Iran has defaulted on commitments related to its nuclear program?

Maybe Atomstroiexport is the Russian firm whose business methods are completely above reproach. You know - the exception that proves the rule. But if so, the reporting has not mentioned it.

I'm just saying...

Update: Bonus link - are we trying to lull the Iranians into a false sense of security?