Is it necessary to repay a debt to a person after their death if they were deceitful in the transaction and their heirs are unaware of the debt?

The most prudent course is to settle the debt by paying it to the heirs of its owner, or to forgive it, in compliance with the Almighty's saying: (And that you pardon is nearer to piety) and His saying: (And those who suppress anger and pardon people). If you are not inclined to do so and wish to claim your right, then it is necessary to inform the heirs so they may reduce the price by the amount of the deception, or to refer the matter to the judiciary.