Is it permissible to expose the deviance of one who sells his religion for worldly gain and issues fatwas that contradict Sharia, and is that considered backbiting, and is his testimony acceptable, and does his deeming lawful or unlawful what Allah has forbidden make him an apostate if he believes the opposite of what he says?

It is forbidden to accuse a Muslim of having sold his religion for worldly gain without decisive proof, especially if he is a student of sacred knowledge. The default assumption for Muslims is soundness, and the mufti (definition) (issuer of religious edicts) may be an excusable mujtahid (one who exercises independent judgment). One must verify before slandering anyone, but clarifying an error, if it is confirmed, is important. Criticism must be conducted with politeness and respect. If a scholar conceals the truth and issues a fatwa (definition) (religious edict) based on falsehood, he is in grave danger. Ibn al-Qayyim described the consequence for one who conceals the truth and issues a fatwa contrary to it as the eradication of the blessings of his knowledge, his religion, and his worldly life, and that Allah will clothe him in the garment of disgrace, contempt, and shame. As for one who errs in his fatwa due to interpretation or following someone whose تقليد (emulation) is permissible, there is no sin upon him.