Sayyiduna Ali (may Allah be pleased with him) said: If the pledged item is worth more (than the debt) and it is destroyed, it goes in exchange for that debt; and if it is not destroyed, the excess value will be returned. What Khilas narrated from Sayyiduna Ali (may Allah be pleased with him) he took from a written sheet (sahifah); this was said by Yahya bin Ma'in (may Allah have mercy on him) and others. It is narrated from Sayyiduna Ali (may Allah be pleased with him), without qualification, that each of the two parties returns the excess to the other.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الرهن / 11229