Does sin attach to a former employee who worked in a company that practiced deceit and falsehood in its real estate transactions, and participated in that, then repented with a sincere repentance? And is it obligatory for him to return the rights to their owners, and how can that be done with the impossibility of reaching some of them, or due to his inability to remember the exact amounts, especially since some of those affected were content with the dealings and did not express any dissatisfaction? And does anything accrue to the owner of the company if he repents?

Deceit, fraud, and lying to customers are forbidden acts, and the money acquired through such tricks must be returned unless its owners absolve the one who took it. Whoever aids in a sin is a partner in it. The door of repentance is open for whoever repents with sincere repentance, for Allah Almighty accepts repentance and replaces evil deeds with good deeds for whoever repents, believes, and does righteous deeds, as He, the Almighty, says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" Repentance for the rights of people is not complete except by restoring their rights to them or seeking their forgiveness. If it is impossible to reach the rightful owner, then give it in charity on their behalf. You are not obliged to guarantee what was taken without your involvement, but you are obliged to guarantee what you took or participated in taking.