Sanhedrin 33A
Rav Chisada says when to you reverse a judgment -- when the judge has not taken and given with his own hands.. [That is the Mishna in Sanhedrin which says you reverse a judgment either for merit or for guilt in monetary issues. The mishna in Bechorot which says you do not reverse a judgment is when the judge has taken from one party and given to the other with his own hands. In that case the judge himself must pay.
So to rav chisad in Sanhedrin the judge did nothing and the judgment is reversed.
The Talmud asks on rav chisda this cant work in the case a judge said you are patur [not obligated] because in such a case no money changed hands.]

Rashi says something here very strange. He says in the case that the judge did not do it but the defendant himself gave the money to the plaintiff. This seems to be completely against the whole idea of the Mishna . the Mishna is telling us the difference between laws of money and laws of death penalty issues. in one case you reverse the judgment only for merit and in the other you reverse the judgment for merit or not. Rashi  definitely does not fit the Mishna.. From here we have a proof of revival of the dead from the Torah. sorry for the bad joke.

The answer is that we have established to rav chisda that when teh judge says patur ata -you are not obligated that is the same as taking with his own hands in which case the judgment stays fixed. So teh mishna in sanhedrin can only b e talking about the other case in which the judge says you must pay. and then we discovered that he made a mistake and in that case the judgment is reverses. So then the question rashi is thinking about is why does the mishna then say "Either for  having to pay or not"--("lechov or zechut ") we are already telling the plaintiff that he is not obligated. so who is the mishna talking about when it says lechov? Clearly the other person. So if he did not received any money how can it be lechov--that he has to pay.