What is the ruling on my continuing to teach the Qur’an at an association where I discovered its owner’s dishonesty and his being alone with girls, knowing that my work is sincerely for the sake of Allah, and I do not wish to speak in a way that would tarnish his reputation?

There is no need to abandon the society because of this man's actions, such as taking suspicious donations and engaging in dubious financial transactions, or his seclusion with a foreign girl. Rather, one should continue in it as much as possible, for good is not forsaken due to evil, and Allah Almighty has benefited many people through this circle. You will not be burdened by the sin of his actions, for his burden is his own. However, one should strive to advise him and attempt to rectify his situation, and one should not remain silent when capable of denouncing it.