Is it permissible to pray behind a disbeliever who denies a part of the Qur’an, and if I prayed behind him out of interpretation, is it obligatory to repeat the prayer?

One must be extremely cautious about rushing to declare a Muslim an infidel (kafir). It is impermissible to rule someone an infidel without ensuring that the conditions for excommunication (takfir) are met and its impediments are absent, for the accused might be ignorant or have an interpretive understanding, in which case they would be excused.

If the repentance of one who was ruled an infidel is established, then it is permissible to pray behind him, and he is to be considered a Muslim. The testimony of a witness is not rejected simply because he lets his garment hang below his ankles (musbil) or shaves his beard (haliq), for he might believe it permissible, following some scholars.