Is money earned from selling blank invoices to a brother who works as a purchasing agent considered illicit wealth, knowing that the seller was unaware of its impermissibility at the time of sale?

It is impermissible to purchase invoices that involve falsification, deceit, and fraud, due to the presence of false speech and deception therein. Allah the Almighty said: "(So shun false speech)" [Quran 22:30]. And the Prophet, peace be upon him, said: "Whoever cheats is not one of us." Falsification includes deceit in documents and records, and counterfeiting signatures with the intention of deception. The money acquired from this is unlawful because it is in exchange for an illicit benefit. However, whoever acquired it unknowingly of its prohibition and then repented, is not obligated to dispose of it.