If I'm not mistaken, Iran under sanctions had been selling oil to China for euros. In their hatred for America, they went so far as to have set up a bourse on the island of Kish in the Persian Gulf. The bourse had been intended to operate in various currencies other than the US dollar (primarily the euro and Iranian rial)...
Don't know what's become of this enterprise, though
So, Iran now competes with Russia in sell oil to China. But Iran's cash goes to Russia anyway to buy missile systems (under the latest agreement, for "domestic" use only). But where is China getting excess cash to buy Iranian oil? From it's trade surplus with the US. Obama's world trade policies are working well.
You dramatically forget that the US is not the only pebble on the beach. Since China allegedly buys Iranian oil for euros, it gets them from its trade surplus with the EU...
Yeah, I understand the pain in Obama's ass