Reference for Bava Kamma 86:19

ולריש לקיש נמי נימא מדעבד אם עבד לא דריש כופר אם כופר נמי לא דריש

[to kill] just as <i>kofer</i> [is paid] where there was intention. Abaye however said to him: If so, the same could now surely also be argued in the case of a slave: viz.: What is taught by [the expression] <i>If a slave</i>?<span class="x" onmousemove="('comment',' Ex. XXI, 30; for it is surely neither an optional nor a conditional liability. ');">¹⁵</span>

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