Musannaf Ibn Abi Shayba 13:2644

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Yunus, from al-Hasan, that he used to say: If a man buys goods on the condition that he has the option [to return them], and they perish while in his possession, he said: If he had specified the price, then he is a guarantor for it. And if he had not specified the price, then he is a trustee regarding it.