One of the differences between dinei mamonot (monetary cases) and dinei nefashot (capital cases) that is taught in the Mishna (32a) is whether we will reopen a case when someone suggests that new evidence is available. In dinei mamonot we will always be willing to reopen a case; in dinei nefashot we will only do so if the original decision found that the defendant was guilty. If he had been found innocent we will not reconsider the case, even if new evidence is forthcoming.
The Gemara on today’s daf suggests that the source for both of these laws is based on a passage in Sefer Shemot (23:7) that says ve-naki ve-tzadik al taharog – “do not kill an innocent or a righteous person.” Thus, the reason we will reopen a capital case if the defendant had been found guilty is because of the command of the Torah – do not kill an innocent person. This obligates the court to do all that is in its power to ascertain whether someone is innocent. The reason we will not reopen a case where the defendant has been found not guilty is because the Torah also commands not to kill a righteous person, which is understood to mean that someone declared righteous cannot be killed.
Rabbi Ḥiyya bar Abba quotes Rabbi Yoḥanan as limiting the law that we will not recall a decision of “not guilty” to cases where there was a true error in judgment. If the law was obvious, however (i.e. it was clearly written in the Torah) then we cannot accept such a ruling, and the case would be retried, since the ruling had no basis in halakha whatsoever.
The expression used by the Gemara to describe a law that is clearly written in the Torah is davar she-ha-tzedokim modim bo – something that even the Sadducees agree to. The Sadducees were known to reject many of the rabbinic interpretations and oral traditions, accepting only laws that appeared in the written Torah.