Once a presidential pardon is granted and finalized, it generally cannot be overturned
. The U.S. Constitution gives the President broad authority to grant pardons for federal offenses, and this power is not subject to legislative or judicial review. The Supreme Court has affirmed the broad and unlimited nature of this power, except in cases of impeachment.
Why are pardons considered final?

Constitutional Authority: The power to grant pardons is vested solely in the President by the Constitution. This separation of powers means other branches of government cannot interfere with a President's pardon decision.
Supreme Court Precedent: The Supreme Court has consistently held that the President's pardon power is not subject to legislative or judicial control. Once a pardon is complete, there's no power to revoke it.

Are there any exceptions or gray areas?

Undelivered Pardons: In one historical instance, a federal court ruled that a pardon could be revoked before it was formally delivered to the recipient.
Potential Challenges: While overturning a pardon is highly unlikely, legal challenges have been raised regarding a pardon's scope and constitutionality in certain circumstances. However, these are rare and have not led to the overturning of a pardon.

In summary, a presidential pardon, once finalized, is a powerful act of executive clemency that generally cannot be reversed. As stated by legal experts in an Al Jazeera article, "Pardons are final and irrevocable."