Arachin, Chapter 3, Mishnah 3. This chapter presents a series of laws in which the Torah sets a fixed sum rather than an assessed value, and in each case the Mishnah shows that the very same fixed sum can work out to be a leniency in one situation and a stringency in another. Our mishnah applies that pattern to the case of a dangerous ox that killed a Canaanite slave.
Some background is needed. An animal that has already gored on three occasions becomes a shor hamuad, meaning its owner has been formally put on notice that his beast is dangerous. Should such an animal kill an eved k'naani, a Canaanite slave, the Torah imposes a payment of thirty sela'im upon the animal's owner, to be given to the slave's master. That figure never moves. What the slave might have brought on the market carries no weight in the calculation at all.
Our mishnah begins with the case: "beshor hamuad," a forewarned ox, "shehemit et haeved," which caused the death of a slave. In such a case, says the Mishnah, the law contains "l'hakel ul'hachmir," a side that is lenient and a side that is stringent.
"Keitzad," how so? The Mishnah answers with the word "echad," indicating that one single rule covers two opposite situations. The victim may have been "hana'eh sheba'avadim," the finest of slaves, who would have commanded a steep price from a buyer, or "hakaur sheba'avadim," the least appealing of them, worth very little. The verdict does not shift: "noten shloshim sela," thirty sela'im are handed to the master, not a coin above and not a coin below.
What if the victim was "ben chorin," a free man? Then the rule is altogether different: "noten et shovyo," he pays that man's worth, that is, the price the deceased would have brought had he been put up for sale as a slave, and the money is given to his heirs. No set sum governs here; the whole matter rests on appraisal.
"Chaval baze u'vaze," should the animal have merely injured one of them, slave or free man alike, "meshalem nezek shalem," the owner covers the damage in full. That is worked out by valuing the victim as though he were being offered for sale, comparing his price while whole against his price now that he is maimed, and paying the difference. Once again the sum is arrived at by estimation, not by a rate written in advance.
Now we can see precisely where the leniency and the stringency lie. Place the flat thirty sela'im paid for a slave alongside the appraised sums paid for killing a free man or for injuring anyone at all. Suppose the slave was a valuable one who would have sold for well above thirty sela'im: here the flat figure works in the defendant's favor, since he parts with less than the loss he caused. Suppose instead the slave was a cheap one worth well under thirty sela'im: that identical figure now weighs against him, since he must hand over more than the master actually lost.