Musannaf Ibn Abi Shayba 13:2817

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Zam'ah, from Ibn Tawus, from his father, who said: If he purchases a sale on the condition that he has the option [to cancel] it, and he sells it before his companion comes, then his sale is valid, and it is lawful for him.