Imam al-Shafi'i (may Allah have mercy on him) said: "Freeing a neck (slave) who is on a religion other than Islam will not suffice him (in the expiation for zihar), because Allah Almighty says concerning killing: 'then the freeing of a believing neck (slave)' [Surah al-Nisa: 92].
So Allah Almighty's condition (of faith) for the neck in the expiation for killing, since it is an expiation, is like evidence, and Allah knows best, that no neck will suffice in any expiation except a believing one; just as Allah Almighty stipulated uprightness ('adalah) for testimony in two places and mentioned witnesses without qualification in three places, and since all of them are testimonies, we inferred from this that the testimonies mentioned without qualification are, if Allah wills, of the same meaning as the one for which the condition was laid down."
He said: "And Allah Almighty returned the wealth of the Muslims to the Muslims, not to the polytheists."
He said: "And I prefer for him that he free only an adult believing (slave woman); and if she is non-Arab and describes (the beliefs of) Islam, (freeing her) will suffice him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الظهار / Q15266