Or HaChaim on Exodus 21:20/אור החיים על שמות כ״א:כ׳

1 בשבט, with a rod, etc. Maimonides writes in chapter 2,14 of Hilchot Rotzeach: "I believe that the word "rod" is to be taken literally. If the master hit the slave with a knife, a sword, a stone or a fist, or something similar, and the court judged that the slave would die from the result of such an attack even after a full year, the master is subject to execution." I do not understand why the master should be executed if he used an instrument which is normally not lethal if the victim did not survive for 24 hours. In such a situation the assailant is not guilty of the death penalty even if he struck a person who is not his slave, i.e. not his personal property. I believe that the reason the Torah writes בשבט with a rod, is that if the master's actions prove that all he wanted to do was to discipline his slave, something normally done with a rod, G'd allowed him a 24 hour period before he would be considered guilty of murder seeing that the slave is his personal property and one does not destroy one's personal property on purpose. If the instrument used to inflict fatal injuries on the slave was one that is not normally employed when one wants to discipline someone but a weapon used when one is about to kill a person, such as stabbing the slave in his belly with a sword, the Torah does not grant the master a reprieve of 24 hours during which survival of his slave may save him from a murder charge. He will be guilty of murder even if death occurs a year after the stabbing, for instance.