What is the ruling on one who says that the Qur'an existing now is not the one that was revealed to the Messenger, may Allah bless him and grant him peace, knowing that he is a Muslim, educated, and writes in newspapers (defending Islam, as he claims), and is it permissible to deal with him if he is a merchant?

The sacred texts clearly indicate that the Holy Quran is preserved from alteration and distortion. As for dealing with someone who claims the Quran has been distorted, there is no harm in it, following the example of the Prophet (peace and blessings be upon him) who dealt with idolaters and Jews. However, dealing with such a person must be accompanied by extreme caution regarding their deviant ideas, along with advising them and warning people against their falsehoods, as this constitutes sincere advice to Muslims.