NSA cannot offer a valid immunity deal because it lacks authority for plea bargaining. What Snowden needs is legal immunity, not amnesty. Amnesty is not a legal immunity. Immunity cannot be withdrawn unless it was made conditional.

A conditional immunity may be withdrawn if, for example, Snowden failed to return some documents if return of documents is a condition of immunity. If immunity is withdrawn, then Snowden could be prosecuted.

The U.S. Department of Justice would have to make the offer of immunity. And there are different degrees of immunity, including partial immunity or “use immunity” and complete immunity or “transactional immunity”. Snowden would have to demand complete immunity and get it in writing from the Attorney General.