מדרש על מכות 9:4

"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).