you can't trust any deal that Obama made. Any deal that Obama made with congressional approval would be binding. The GOP had grave concerns about that deal, and since it bypassed constitutional congressioanl approval, they were letting the Iraninas know that it would be reversed if power changed party hands. Because of that letter, the world was not surprised when we pulled out after the Trump election, and the other countries all clearly know that no agreement from the US is a real deal unless it is also approved by congress.
The constitution is clear on this, and it was another example of Obama's presidential over reach. Now, it works the other way against Trump if he tries to be a cowboy without congressional approval. If that happens, then nobody on the Right has a right to complain. But that hasn't happened yet. I don't think that it will.