Musannaf Ibn Abi Shayba 13:2844

Abu Bakr narrated to us, he said: Abu ‘Asim narrated to us, from ‘Imran al-Qattan, from Matar, from ‘Ata’, from ‘Ubayd ibn ‘Umayr, from ‘Umar, who said: If the collateral is more than what it was pledged for, then he is a trustee regarding the surplus. And if it is less, he returns [the difference] to him.