Is it permissible for a Muslim to work for a company that specializes in transporting pianos, knowing that his work is restricted to transporting pianos only for disbelievers? Is this considered cooperation in sin? And what is the ruling if he happens to find a piano among the household furniture he is transporting—is he permitted to transport it in this case?

The piano is a forbidden musical instrument. It is not permissible to sell it, rent it, carry it, or assist in its use, due to the Almighty's saying: (And cooperate in righteousness and piety, but do not cooperate in sin and aggression). Therefore, working for a company that transports pianos is forbidden. And whoever is hired to transport household furniture and finds a musical instrument therein, it is not permissible for him to carry it, because that is assisting in disobedience. And whoever gives up something for the sake of Allah, Allah will compensate him with something better.