What kind of Mum is discussed?
Moshav Zekenim citing Toras Kohanim: We learn from a Klal u'Ferat u'Chlal. [Our verse is a Klal]; Ayin Tachas Ayin is a Prat. Ka'asher Yiten Mum is another Klal. Just like the Prat is a permanent blemish in an extremal, exposed limb (so one is liable for such wounds). Similarly, an Eved Kena'ani goes free (if his master knocked out such a limb). 1
Moshav Zekenim: If so, if the master struck his slave's hand and it will recover, just like he does not go free, we should say that one does not pay Sheves Gedolah (decrease in what one would pay to buy such a person for a slave, based on his lifelong loss of earning potential). Abaye says that he pays Sheves Gedolah and Ketanah (compensation for inability to do even work that does not require his hand at all, until he recovers! He could also ask about other limbs that do not set a slave free, e.g. he broke his arm. - PF)