Does the Iran-China pact give the USA an option for slapping China with a large scale sanction?

Of course. How many countries is the U.S. currently sanctioning, for one reason or another? The Canadian mounties arrested Meng Wanzhou - the daughter of Huawei's CEO - on behalf of the U.S. government, which claims that she violated existing sanctions in doing business with Iran.

Given the arrogance of the U.S. government, officials could probably sanction Switzerland for selling cheese that's full of holes.

The ramifications of such sanctions is another story. There is a limit to how many sanctions even the U.S. can impose. In addition, many countries are fed up with U.S. intimidations, of which sanctions are but a part. Finally, remember that China is capable of retaliating - in more ways than one.

Most countries are afraid to defy the U.S. When countries like China and Iran defy the U.S. and get away with it, it can encourage other countries to do likewise.

Again, the U.S. can do a lot of things. It seems like Donald Trump, in particular, does whatever he wants. However, it looks like the price tag might be more than he bargained for.

A nation has the right not to trade with anybody they don't like. They even have the right not to trade with anybody who trades with anybody they don't like. No pretext is necessary, really. Passing such sanctions on a whim might run afoul of international trade treaties, but a nation has the right not to be a member of the WTO, or not to sign any bilateral or multilateral investment protection treaties.

So the United States have to ask themselves if either direct or secondary sanctions hurt them or China more, politically and economically. And if they want the rest of the world to match their sanctions, they will have to make the political case.

The US has a tendency to codify their sanctions decision in domestic law, but that's just domestic law, for the US to alter as they see fit.