עַיִן תַּחַת עַיִן "An eye for an eye" [Mishpatim 21:24]
The term "eye for an eye" explain Chazal [Bava Kamma 84a] is not meant to be taken literally - one who causes another the loss of an eye is not punished by having to lose his own eye. Rather, it means that the responsible party must pay the monetary value of an eye.
Chazal's interpretation of this halachah, said the Vilna Gaon, is alluded to in the words of the verse. Why does the verse state "Ayin tachas ayin" - which literally means "an eye beneath an eye" - and not "Ayin be'ad ayin" - which means "eye for an eye"?
The Torah, explained the Gaon, is hinting to us that in order to discover the true meaning of the verse, we must look at what is "beneath" the ayin, that is the letters that follow the word "ayin" עין:
The letter ayin ע is followed by the letter pei פ
The letter yud י is followed by the letter kaf כּ
The letter nun is ן followed by the letter samech ס
These letters form the word keseph - כּסף - money !
Source: Rabbi Yisrael Bronstein