What is the ruling on working in a commercial project largely funded by money that a brother disposed of without the owner's permission, and how can the brother repent and clear his conscience, and what is required of the brothers to ensure the lawfulness of their money?

It is forbidden to dispose of a trust (wadi'ah) without the owner's permission. Whoever does so is a usurper (ghasib) and must repent and return the trust. The loss of a part of the trust obliges your brother to compensate its owner for it, because he transgressed by disposing of it. As for the profit from the part of the trust that entered the project, scholars have differed on this: The Hanbalis believe it belongs to the owner of the money, while the Malikis and Shafi'is believe it belongs to the usurper. Abu Hanifa believes it should be given in charity.

Your brother must promptly return the full trust without delay, even if it leads to the dissolution of his partnership with you. He must ask for forgiveness from the owner of the trust, estimate the money that entered the company, and give its profit to the owner of the trust or agree with him on a part of it, in order to purify his money from the unlawful. You must advise him to get rid of what is unlawful and to repent.