There are 2 separate issues here: what happens to such a President and what happens to the person who has been pardoned.
What happens to the person who has been pardoned?
While at least one attempt at reversing a pardon has been discussed in recent history (Clinton's pardon of Mark Rich), there is no case of a pardon that has been reversed without the wishes of the person who was to be pardoned.
There were 2 SCOTUS cases which decided that a pardon is a form of clemency rather than an act of overturning a judgement.
- United States v. Wilson, 32 U.S. (7 Pet.) 150 (1833) established that if a person does not accept a conditional pardon, then it is not in effect.
- Burdick v. United States, 236 U.S. 79 (1915) decided that if a person does not accept an unconditional pardon, then the pardon is not in effect. Burdick specifically did not want to accept the pardon because his contention was that accepting it would be tantamount to admitting guilt and would strip him of his 5th amendment right to not incriminate himself.
There is very little case law surrounding the understanding of the power to pardon, so it may help to simply list the relevant instances of considerations and available opinions.
Both (fmr President) Nixon and (fmr Sec of Defense) Weinberger were pardoned without ever being tried for the crimes for which they were pardoned.
Even after Burdick v US, the issue of whether a pardon does amount to a formal admission of guilt remains controversial (i.e., not fully settled legally).
In Nixon v US (not to be confused with the more famous US v Nixon), the court referred to Black's Law dictionary, rather than to the previous 2 opinions, to state that a pardon does not overturn a "guilty" judgement but rather provides a clemency.
It is established that all punishments (jail time or fines), that one would receive for the crime, would not be applied if the person is pardoned.
However, it is not established, for example, if the the presumption of guilt that goes with the accepting of a pardon
- counts as a "strike" for the purposes of "3 strike" laws (because of no precedent);
- would result in a requirement to continue registering as a sex offender in case the crime was a sex crime (because of no precedent).
The current DOJ FAQ states that a pardon removes "civil disabilities" such as restrictions on the right to
- hold state or local office
- sit on a jury
It is widely claimed that a President cannot pardon anyone for state crimes. However, as far as I know, it's never been attempted.
The current interpretation of the US Constitution's Supremacy Clause is that states cannot interfere with the proceedings of the Federal government. So should a President attempt to pardon anyone for a state crime, it would (almost certainly) result in a court challenge. Any claims, that the outcome of such a challenge would result one way or another, are (by definition) nothing but a speculation.
Further, if a pardon does remove the civil disability of not being able to hold a state office, then it does remove punitive consequences of some states' laws. Which may potentially bolster the claim that a President may pardon a state crime. But again, this is a pure speculation and there are plenty of good arguments to be made against such a possibility.
What happens to the President who has taken a bribe?
That having been said, no official act performed by a President is automatically reversed if the act is found to have been done corruptly (in exchange for a bribe or any other personal consideration contravening his oath of office).
However, the Congress has the enumerated power to impeach a President and remove him from the office if he is found to have taken a bribe. Removing him from the office does not, in itself, reverse any of his official actions (including the ones performed corruptly).
The Congress, however, does not have the power to reverse pardons. Nor can it create such a power with a legislature. For Congress to gain such a power would require a constitutional amendment.