Midras Ulangan 19:21: Sifrei Devarim

"to do to his brother": From here they ruled: Edim zomemin are not killed unless the verdict has been pronounced (to kill the defendant). For the Sadducees were wont to say: Scheming witnesses are not killed unless the defendant is killed, it being written (Ibid. 21) "a soul for a soul" — to which the sages replied: But is it not written "Then you shall do to him as he schemed to do to his brother" — but his brother is (still) living? If so, why is it written "a soul for a soul"? For I might think that they are killed as soon as their (the refuters') testimony is accepted. It is, therefore, written "a soul for a soul" — they are not killed unless the verdict has been pronounced (to kill the defendant).

R. Yossi Haglili says: Whence is it derived that one should not go out to war unless he has hands, feet, eyes, and teeth? From (the juxtaposition of (19:21) "Your eyes shall not pity; a soul for a soul, an eye for an eye, a tooth for a tooth, a hand for a hand, a foot for a foot" with (20:1) "when you go out to war against your foes."

R. Yehudah says: It is written here "you shall have no pity," and elsewhere (Ibid. 19:21) "You shall have no pity." Just as there, monetary compensation (is indicated), here, too, monetary compensation (is understood).