Well, the agents lawyer is arguing that the NDA is unconstitutional and he will challenge it. Right now its in the hands of the DOJ to decide. What's interesting about this, is that he can surely refer this to the courts, and, even if he is not allowed to testify, he could write a book about what he knows, even do so as a ghost writer in the form of a fictional book.
This would ensure at least what is known is out there for all to understand. I found it interesting also that Clinton claims this is all a conspiracy drummed up by the "alt right" meanwhile, it was left wing papers that broke this story, was it not? Beginning with the NY Times if Im not mistaken.