Is it permissible to benefit from volumes of Qur’anic exegesis (Tafsir) that were gifted to me by a person with whom I had an unlawful relationship, after my repentance, or must they be returned to him?

If the gift of the book is a pure donation, not a compensation for the illicit relationship, then there is no harm in benefiting from it. However, if it is a compensation for it, then it is not permissible to benefit from it, and it is not obligatory to return it to him. Rather, it should be given to someone who can benefit from it, and be spent on the welfare of Muslims, similar to the harlot's dowry and the soothsayer's fee.