Is working as an accountant for a businessman who engages in usurious transactions, sells materials designated for manufacturing in the market, and practices forward selling by selling goods he has not yet received, considered cooperating in sin? And is the salary received from this work lawful or unlawful?

Working with someone who engages in cheating and illicit transactions falls under cooperating in sin, which is forbidden by the Almighty's saying: "And do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty." As for working with him in permissible matters free from cheating, there is no harm in it. However, it is preferable to avoid someone whose condition is such and to seek other work, based on the hadith: "Leave that which makes you doubt for that which does not make you doubt."

It is absolutely impermissible to sell goods before taking possession of them, according to the majority of scholars. Possession varies depending on the item. However, the Malikis permitted selling goods before taking possession, as long as they were not food acquired through an exchange. The opinion of the majority is more sound, based on the hadith of Ibn Abbas, may Allah be pleased with them both: "As for what the Prophet, peace and blessings be upon him, forbade, it is food that it should not be sold until it is taken possession of." Therefore, your companion must refrain from selling goods before taking possession of them.