Musannaf Ibn Abi Shayba 13:2098

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Ibn Yunus, that there were two men who had a slave between them, and they emancipated him on the condition that he serve them as long as they lived. Then one of them purchased the share of his companion from the other. Ibn Sirin was asked about that, and he saw no harm in it.