If I am the owner of a business which is haraam and if I sell that business to someone, would I keep incurring sin after selling the business if the person who I sold the business to operates it and grows the business more than I did?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.

A person is not permitted to sell what is unlawful and shall be considered sinful if he does. It is well-established in Shari`ah (Islamic law) that helping others disobey Allah is forbidden; therefore, if a person helps another do a forbidden act, he shall be considered participating in this sin, whether this sin occurs through selling or any other act. This is in accordance with the saying of Allah, the Almighty (which means): {And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty.} [Quran 5: 2]

As for the buyer’s work in managing and developing the unlawful project, he shall be responsible for his earnings and shall be held accountable for his sin, although you are responsible for the sin of selling the unlawful project to him, from the very beginning. However, when a person sincerely repents to Allah, Allah will accept his repentance and wipe out the sin from which he has repented.

Allah knows best.