Quran and Hadith make no distinction between a Muslim slave and a non-Muslim slave. The only difference is that according to one hadith, freeing a Muslim slave can save a person from hellfire. That does not mean that freeing non-Muslims is not meritorious or is not recommended for expiation of sins. Since, later, a law was established that no free Muslim would be taken as slave, limiting emancipation to Muslims would have been meaningless.
Verse 47:4 of Quran clearly suggests that non-Muslim captives who were made slaves can be freed as a “favor.” In the Battles of Hunain and Bani Mustaliq, non-Muslim slaves were freed. Tabari also lists several non-Muslims who were bought and freed by Prophet Muhammad (s.a.w) or freed when they were captured after a military campaign and they fell in his lot. People who were freed by the Prophet (s.a.w) included Yasar, Abu Muwayhibah, and Abu Kabshah, among others. Caliph Umar bin Abd al-Aziz had ordered that non-Muslim subjects, if taken prisoners by an enemy should be as much ransomed and liberated on state expenses as any Muslim subject [for reference see Ibn Sa’ad’s al-Tabaqat al-Kubra, Volume 5, pages 26, 272. For secondary reference see “Non-Muslims Under Sharia’ah Law” by Abdul Rehman Doi).
How About Verse 4:92 which states:
[4.92] And it does not behoove a believer to kill a believer except by mistake, and whoever kills a believer by mistake, he should FREE A BELIEVING slave, and blood-money should be paid to his people unless they remit it as alms; but if he be from a tribe hostile to you and he is a believer, the freeing of a BELIEVING SLAVE (suffices), and if he is from a tribe between whom and you there is a covenant, the blood-money should be paid to his people along with the freeing of a BELIEVING SLAVE; but he who cannot find (a slave) should fast for two months successively: a penance from Allah, and Allah is Knowing, Wise.
The verse deals with a specific condition in which a Muslim is found guilty of manslaughter for killing another Muslim. In the verse prior to this, it is clear that what is being referred to is unintentional killing of a Muslim, who may or may not be a hypocrite or who may be living in enemy territory. And if life of a Muslim was taken without proper justification, it was only right that another Muslim benefits and he/she gains freedom. Freeing of a Muslim slave as a compensatory corrective measure does not in any way negate the general principle that freeing of slaves, regardless of their faith, is a meritorious act. In other verses of the Quran, expiation of other sins is achievable through freeing of a slave, and no mention is made whether the slave has to be a Muslim or not. See Verses 90:13, 5:89, 58:03, 9:60, 2:177, 24:33.
To further clarify, in case of a non-Muslim victim of manslaughter at the hands of a Muslim or vice versa, other punishments or penalties may be given. The verse in question was strictly dealing with manslaughter of a Muslim by another Muslim in times of conflict, and under normal circumstances a judge should come up with proper verdict in other cases, including freeing of a non-Muslim slave, if possible. Since slavery no longer exists, other measures may be prescribed to inculcate repentance. Regarding protection of non-Muslims, the Prophet once said “Their property is like our property and their blood is like our blood” (see Mishkat al-Masabih, Ch. 25, Hadith 10). Caliph Ali (r.a.) had also stated, “The blood of a Dhimmi (protected non-Muslim) is like the blood of a Muslim, his goods and chattels are like those of the Muslims.”
How About This Hadith which States:
Book 009, Number 3604:
Abu Huraira (Allah be pleased with him) reported: I heard Allah’s Messenger (may peace be upon him) as saying: He who emancipates a BELIEVING SLAVE. Allah will set free from Fire his every limb for every limb of his (slave’s), even his private parts for his.
The Hadith simply implies that freeing a Muslim slave is such a supererogatory act that a person can save himself from hellfire. It does not mean that freeing a non-Muslim slave does not have its own rewards. Most of the other Ahadith and Quran do not make a distinction between a Muslim and a non-Muslim slave when manumission of a slave is recommended for expiation of sins.