Is it permissible for an Islamic preacher to exploit the giving of lessons in the mosque to promote his merchandise, and is it permissible for the officials to allow him to do so?

There is no harm in enabling the preacher to deliver his lessons and sermons if people benefit from his preaching, provided he is prevented from exploiting the lessons to promote his trade, so that his preaching may be purely for the sake of Allah, and so that the image of da’wah (inviting to Islam) is not tarnished, and to keep mosques free from commercial advertising. Sheikh Ibn ‘Uthaymeen mentioned that announcing acts of obedience inside the mosque is permissible, but announcing worldly matters is not permissible inside it; rather, it should be done outside.