Or HaChaim on Exodus 23:7/אור החיים על שמות כ״ג:ז׳

1 כי לא אצדיק רשע, "for I will not acquit the wicked." According to Sanhedrin 33 the term נקי refers to someone who testifies that he knows of evidence in favour of a person who has been convicted of a certain misdemeanour. In such a case the trial is re-opened. The expression צדיק in our verse, on the other hand, refers to a person against whom accusations are levelled after he has been acquitted. The Torah states therefore, that once a person has been acquitted of a certain crime he cannot be tried for it a second time. G'd reassures the reader that in the event the acquittal by the court was based on an error, He, G'd, will not allow the person who parades as a צדיק to go scot free. The Torah does not want us to think that seeing a person has been found guilty, "how can the verdict be overturned by fresh evidence?" The Torah answers simply: If the original conviction was fair and corresponded to the facts, I, G'd, will not allow such a person to get away with it even if he is found innocent in his second trial. The testimony on his behalf will not ultimately result in a perversion of justice. If, on the other hand, the original conviction was based on error, why should the person who can offer testimony on behalf of the convicted be ignored? Concerning the person who had been acquitted and against whom fresh evidence has been found, the Torah simply states that the fact that the court declared someone innocent does not necessarily mean that he is innocent in the eyes of G'd too. G'd has His own court and it is quite inconceivable that G'd would seal a decree by a human court which is erroneous. We are taught in Ketuvot 21 that it is forbidden for anyone to sign a document which appears to be fraudulent. You may well ask why we do not apply the same principle, i.e. that a court erred, and re-open the trial of someone who had been freed before new evidence against him had come to light? I believe we can best answer this by recalling that G'd "regrets" something evil on occasion. He does not, however, "regret" a decision which was favourable. As a result of this consideration, if a person has once been acquitted he is not subjected to a trial again for the same alleged offence. When a person has been found guilty in his first trial, the suffering he endured until he was finally exonerated in a second trial may even approximate the suffering experienced while he faced execution for a sin not comitted. He may therefore ultimately be exonerated by a Heavenly tribunal even for sins committed for which he had not been tried at all.