Is it permissible to sell the device of a debtor who does not pay his debt, knowing that the device is stolen and its original owner is unknown, in order to recover part of the debt, even though the value of the device is less than the debt?

It is not permissible to buy stolen goods or to assist in their sale, as they are unlawful wealth that cannot be legitimately owned. Anyone who knows that something is stolen must denounce the thief, order him to repent and return the goods to their owners, or try to return them himself, or inform the responsible authorities. Whoever buys stolen goods knowing them to be stolen has sinned, and the completeness of his repentance is to return them to their owners.

As for the issue of gaining possession (al-Zafar): If a person has a right with another and is unable to reclaim it, it is permissible for him to take the equivalent of his right from the oppressor's wealth if he is safe from discord and does not cause greater harm. Based on this, it is not permissible for you to take the stolen computer in exchange for your money. Rather, you must try to return it to its original owner or their heirs, or hand it over to a just Muslim ruler, or to a group of just Muslims. If this is not possible, then sell it and donate its price to poor Muslims on behalf of its owners. If you hand it over to an unjust or corrupt ruler, your conscience is not cleared.