The United States is not bound in the slightest by Obama’s agreement.
I don’t know what Trump’s plan is, but he’s can do whatever he likes.
To ratify it, he will have to submit it to Congress.
If he doesn’t want to honor it, it’s dead.
That also would be an EXCELLENT PLAN.
The problem is that there really is no “Obama’s agreement”. What there is is an agreement between the UN and Iran which Obama negotiated. When Obama leaves the scene, the UN-Iran deal will remain. At that point, Trump’s only leverage will be whatever leverage he has within/over the UN. There is no deal signed by Obama and Iran for Trump to walk in and tear up. Instead, undoing the deal would mean undoing the UN and the will of the “international community”. That’s a heavy lift, but you get the feeling that that’s where Trump is heading.