Is prayer valid behind an imam who sells and trades in tobacco and cigarettes, when the mosque's regular imam is absent?

Prayer behind this man is valid according to the preponderant opinion, though it is disliked. The man should be advised regarding the impermissibility of trading in tobacco, and that the scholars of the Ummah are unanimous in prohibiting it due to the great harm it contains. He must repent to Allah and seek a lawful livelihood.