Is it permissible to pray behind an imam who recites the Qur’an with the Qahiri letter "Gim" (ج), and what is the ruling on praying behind an imam who says: "Sami‘a Allahu liman hamid" (Allah heard him who praised Him)?

The letter 'jim' (ج) that is generated between a 'ghayn' (غ) and a 'qaf' (ق) is not classical Arabic, and to recite the Qur'an with it is a لحن (لحن: an error in recitation that alters meaning or grammar, or is simply ungrammatical) that must be avoided. If the worshipper's recitation of the 'jim' in this manner constitutes a clear لحن, then it is not permissible to pray behind him unless he is unable to learn the correct pronunciation. As for the saying: "Allah hears the one who praises Him" (سمع الله لمن حمده), it is obligatory according to the Hanbalis, and prayer is invalidated by deliberately omitting it or making a لحن in it. Praying behind someone who deliberately does so is invalid according to them. However, according to the majority of scholars, prayer behind such a person is valid.