Is a person who sells a DJ system by installment, knowing that it will be used in weddings, sinful, and what is the ruling on the money received in advance and the future installments?

If you know or strongly suspect that the buyer will use the machine for an unlawful purpose, it is not permissible for you to sell it to him, based on the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." And whoever aids in a sin is a partner to its perpetrator in sin, as is the case with the cursing of the one who consumes usury, its payer, its scribe, and its two witnesses. Since you were unaware of the ruling, we hope that you will not be held accountable and that the price will be lawful for you, as Shaykh al-Islam Ibn Taymiyyah held that whoever earned unlawful money with the payer's consent and then repented, what has passed is lawful for him. Some scholars consider the sale valid despite its impermissibility, such as selling grapes to one who will turn them into wine.