He left his watch in pledge with the seller, and he did not come to pay off what he owed at the appointed time
Question

I have a store, and one of the customers came and brought something, and he left this watch of his with me for a specific period of time. The appointed time came, and he did not come to me, so I kept the watch and wore it. Is there any sin on me for that?
Praise be to Allah.What you must do is be patient until the owner of the watch comes and gives you what he owes to you. If that is too difficult for you and you do not want to wait, then you can sell it by auction in the marketplace for the best price, then take what is owed to you and keep the rest for its owner. Then if he comes to you some day, you can give him his due. If a long time goes by and you do not know him and you are not able to give him his due, then you can give it in charity to the poor, with the intention of that being on behalf of the owner. This is the way prescribed in Islamic teachings for dealing with this matter."(Shaykh ‘Abd al-‘Azeez ibn Baz (may Allah have mercy on him).
Fatawa Noor ‘ala ad-Darb, 3/1457.