The Vilna Gaon says on this, that the word “tachas” (“under” which seems to be a strange word to use in this instance) alludes to the fact that you pay for an eye rather than having your eye knocked out. How so? He says if you take each of the letters under (“tachas”) עין, you get פכס which when rearranged spell כסף hinting to the din to pay for an eye rather than knock out an eye.
Sheya
says:The Vilna Gaon says on this, that the word “tachas” (“under” which seems to be a strange word to use in this instance) alludes to the fact that you pay for an eye rather than having your eye knocked out. How so? He says if you take each of the letters under (“tachas”) עין, you get פכס which when rearranged spell כסף hinting to the din to pay for an eye rather than knock out an eye.
zvi
says:ayin igros Chazon Ish #81 – ayin chidushei HRH”G R’ Shimon Sofer