Is a Muslim rewarded for a good deed he was forced to do against his will, and is the reward for a good deed done for the sake of Allah withdrawn if its doer later regrets it?

For righteous deeds to be accepted and rewarded, they must be done solely for the sake of Allah the Almighty. This is supported by texts from the Qur'an and Sunnah. There is no reward for one who does not intend by their action the Face of Allah. One who is compelled to perform a righteous deed receives no reward because they did not form the intention nor seek the Face of Allah. As for regretting a righteous deed, this is not among the causes for its reward to be nullified. The general principle is that deeds, once completed, are not invalidated except for reasons explicitly stated in Islamic law.