إذا أعتق المكاتب عن كفارة يمينه أو ظهاره جاز
In the stated legal analysis, emancipating a *mukātab* for expiation of an oath or ẓihār is valid.
ولا يجوز فيهما إعتاق المدبّر وأمّ الولد
The corresponding emancipation of a *mudabbar* or *umm walad* is treated differently and not counted for those expiations.
At first this may appear counterintuitive. The distinction turns on what the obligation to emancipate is meant to accomplish: an effective and complete removal of the legally relevant bondage. An arrangement that has already established a different pathway to freedom may not fulfill that exact act.
The source's lesson is to analyze the legal effect that the operative word requires, rather than assume that every case containing the word 'slave' has identical implications.