Allowing Congress to override a pardon is a change in the law that would be effective only if done by amending the Constitution. A statute purporting to do so would be an unconstitutional infringement of an expressly granted presidential power.

    “The President … shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment.”

    - Article II, Section 2, Clause 1