Congress can't pass a law removing the specific authority that President Trump used in order to stop him, because that would be an ex post facto law. They can only remove the authority from future use. That can be vetoed, and that veto can be overridden.
-PJ
The House and Senate can pass a joint resolution withdrawing the Emergency Declaration. In other words they can pass a bill that says there is no emergency. If that bill is passed the President can veto it requiring a 2/3 majority of both houses to override.