Midrash for Makkot 9:1

We are hereby taught that he is not liable until he falsifies himself — whence they ruled: Witnesses do not become zomemin ("scheming witnesses") until they falsify themselves. How so? A says: I testify that that man killed someone (on a certain day). B. says: How can you say that? That slain man (or that slayer) was with us on that day in this and this place? — These are not zomemin. But if they said: How can you testify thus? You were with us on that day in that place — These are zomemin, and they are killed for their testimony.

"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).