Is it permissible to work for a drug dealer's company, accept gifts and presents from him, and engage in commercial dealings with him?

If the company's activity is in forbidden things, such as drug trafficking, then working for it is forbidden. However, if its activity is permissible and has no relation to forbidden things, then working for it is permissible, and the mere ownership of it by someone who deals in drugs does not prevent that. It is permissible to engage in lawful dealings with someone whose wealth includes both lawful and unlawful components, as long as his wealth is not entirely unlawful, and the transaction is not specifically with his unlawful wealth. Indeed, the Prophet, peace and blessings be upon him, traded with the Jews through buying and selling despite their involvement in usury and gambling, and he gave them Khaybar to cultivate, and bought food from them with a pledge.