What is the ruling if non-Muslims release a person under the condition that he is their slave?

Primary text

If they release him on the condition that he is their slave, Abu al-Khattab states that he has the right to steal, flee, and kill. This is because his status as a slave is a legal ruling (hukm shar'i) that cannot be established by their mere words, and even if it were established, it would not entail safe conduct for him from them, nor for them from him. This is the established position (madhhab) of Shafi'i. If they make him swear an oath regarding this condition, and he was coerced into the oath, his oath is not contracted (effective). If he swore by choice and then broke the oath (hanith), he performs the expiation (kaffarah) for his oath.