The Qur’anic wording on expiation for an oath includes:
«أَوْ تَحْرِيرُ رَقَبَةٍ»
Or freeing an enslaved person. (Al-Ma’idah 5:89)
Here raqabah refers in the classical legal setting to an enslaved person who could be emancipated. The attribute mu’minah is not attached to the noun in this particular text, so it illustrates unrestricted wording.
The point is to identify the qualification actually expressed in a text. This historical example does not authorise enslavement in the present day.