Rashi on Exodus 21:23/רש"י על שמות כ״א:כ״ג

1ואם אסון יהיה AND IF THERE BE ANY FURTHER MISCHIEF — in the case of the woman,

2ונתת נפש תחת נפש THEN THOU SHALT GIVE SOUL FOR SOUL. Our Rabbis differ as to the explanation of the word נפש the first time it occurs here. There are some who say that it actually signifies “life” (i. e. life for life), others say that it means monetary compensation but not literally life, and they say that this must be so because he who intends to kill a certain person and inadvertently kills another instead, (as is the case here), is exempt from the death penalty, and has only to pay to his heirs his value estimating this as though he were sold as a slave in the market (Sanhedrin 79a).