Makkos, Chapter 1, Mishnah 6. The Mishnah teaches a further law regarding scheming witnesses (eidim zomemin): "Ein ha'eidim zomemin neheragin ad sheyigamer hadin" - scheming witnesses are not put to death until the verdict has been completed and the court has issued its ruling on the basis of their testimony. Therefore, if the court found the accused guilty on the strength of their testimony and declared him liable, and afterward the witnesses were contradicted and shown to be liars - at that point, since a verdict had been reached on the basis of their testimony, they are subject to 'ka'asher zamam,' and in a capital case they are put to death in his place.
The view of the Tzedukim:
This point is emphasized because the Tzedukim - those who did not accept the Oral Torah - said: "Ad sheyeihareig" - until he is killed. In their opinion, scheming witnesses are not put to death and are not subject to 'ka'asher zamam' unless the court actually carried out the punishment on the basis of their testimony, and in a case whose penalty is death - not until the accused has actually been executed. Their source: "Shene'emar: nefesh tachas nafesh" - a life in place of a life; only if the man was killed do we take their lives.
The Tosafos Yom Tov notes that the wording of the Mishnah before us is imprecise: "nefesh tachas nafesh" is a verse in Vayikra that does not deal with scheming witnesses, and the correct wording in the Mishnah is "nefesh b'nafesh," an expression used in the passage about scheming witnesses. In any case, the Tzedukim drew the very same idea from those words: a life in place of a life - only after the man against whom they testified has been killed and the sentence carried out do we say that the witnesses are subject to 'ka'asher zamam.'
The response of the Chachamim:
The Chachamim said to them: "V'lo kevar ne'emar: va'asisem lo ka'asher zamam la'asos l'achiv" - we do to these witnesses what they intended to do to their brother, "vaharei achiv kayam" - and behold, his brother is alive. The word "l'achiv" teaches that he is still living, and from here they proved to the Tzedukim that the verse must be speaking of a case in which the accused has not yet been killed.
"V'im kein lamah ne'emar nefesh b'nafesh":
If so, why does the verse say "nefesh tachas nafesh" - or really "nefesh b'nafesh," as the Tzedukim pointed out? This verse comes to teach the law with which we opened: "Yachol mesha'ah shekiblu eidusan yeihareigu" - one might have thought that from the moment the court accepted their testimony, and they were afterward contradicted, the law of 'ka'asher zamam' applies to them and they are put to death on the strength of the mere acceptance of the testimony, without a completed verdict. To this the verse says "nefesh b'nafesh": they are not put to death until there is a completed verdict. And even though the punishment has not yet been carried out, it is considered as though it had been carried out, since we have reached the point at which the man is about to be killed. This is what "nefesh b'nafesh" comes to teach us.
The comment of the Ritva:
Rabbi Akiva Eiger cites here the Ritva, who points out a difficulty in the proof the Chachamim brought against the Tzedukim: from the verse "ka'asher zamam la'asos l'achiv" they proved that the brother is alive, as if the word "brother" is used only of one who is living. The Ritva notes that this is not so, and brings proofs:
Regarding the mitzvah of yibum it says "to establish a name for his brother" - and the brother is dead.
After Nadav and Avihu were killed, it says that they should carry out their brothers who had been killed.
Rather, the Ritva explains that Chazal sometimes make use of a particular verse even though it is not the primary reason. The true reason we do not follow the view of the Tzedukim - who held that the accused would have had to be killed - is from what is stated, "ka'asher zamam," and we learn from it: as he schemed, as he plotted and as he sought to do, and not as actually occurred. This is the true source for this law.
In summary: In this Mishnah we learned that conspiring witnesses are not put to death until the verdict has been reached. The view of the Tzedukim, who expounded "nefesh b'nafesh" and derived from it that the witnesses are punished only after the accused has actually been executed, was rejected on the strength of the verse "ka'asher zamam la'asos l'achiv" - which implies that his brother is still alive; and the verse "nefesh b'nafesh" comes to teach that accepting the testimony is not enough, and a verdict is required. The Ritva added that the essential reason is the precise wording of 'ka'asher zamam' - as he plotted to do, and not as he actually did.