If one forgives someone after being wronged, and then one day later he says, "No, I am not going to forgive him," meaning that he changes his mind, is this valid? Or is the person who wronged him forgiven and is changing his mind not valid? If one behaves in a bad manner or harshly or rudely, meaning that he performs actions which are disliked, and I feel hurt but I forgive him, is this ok, or is forgiveness not required for disliked actions? When punishing a wrongdoer may make him regret and mend his actions, which is better; forgiving or punishing?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

We have highlighted in several fatwas that what appears from the statements of the scholars is that if a person pardons the one who wronged him, he cannot take back that pardon.

Al-Bukhaari wrote a Chapter in his Saheeh which he entitled: "Chapter: If one pardons the wrongdoer for his injustice, then he cannot withdraw that pardon."

As for your second question, it is very strange indeed! Is there forgiveness or pardon in other than injustice that a man dislikes? In every injustice that happens to a person, it is valid for him to pardon the wrongdoer and give up his right that he owes him.

As for your last question, a group of scholars stated that it is better to take revenge against the oppressor rather than pardoning him if pardoning him would lead to the increase of his injustice and evil.

Bareeqah Mahmoodiyyah reads:

"If he is able to take back his right, he may also pardon the wrongdoer, just as when he is unable to take back his right, and this is better than the first pardon (which is the pardon when being unable to take back one’s right) and harder on the soul... However, justice may be better than pardon for a reason that necessitates it, like when pardon would lead the wrongdoer to increase his injustice because of him thinking that not taking revenge against him was due to weakness and inability, and taking back one’s right is a reason to minimize his injustice or eliminate it when the right is in retribution, for instance, and similar necessities, like making him a lesson for others. If one takes back more than his right, then this is injustice." [Abridged]

Ibn ‘Uthaymeen said:

"{…and who pardon the people….} [Quran 3:134]; meaning the people who wronged you, forgive them, because the one who pardons and reconciles gets his reward from Allah. Here the pardon is general, but Allah clarified in another verse: {but whoever pardons and makes reconciliation - his reward is [due] from Allah.} [Quran 42:40]; it means that pardon is not better unless it involves reconciliation. So when a person who is known for transgression, tyranny, and injustice against the people wrongs you, then it is better not to pardon him and take back your right from him, because if you pardon him, he will increase in evil. However, if the person who wronged you seldomly wrongs the people, and he is of little transgression, and it just happened rarely, then it is better to forgive him." [Sharh Riyadh As-Saaliheen]

Allah knows best.