A poor Jew finds a wallet with $700 in it. At his synagogue, he reads a notice saying that a wealthy congregant lost his wallet and is offering a $100 reward for it. He spots the owner and gives him the wallet.
The rich man counts the money and says, "I see you already took your reward."
The poor man answers, "What?"
"This wallet had $800 in it when I lost it."
They begin arguing, and eventually come before the rabbi.
Both state their case. The rich man concludes by saying, "Rabbi, I trust you believe ME."
The rabbi says, "Of course," and the rich man smiles. The poor man is crushed.
Then the rabbi hands the wallet to the poor man.
"What are you doing?!" yells the rich man.
The rabbi answers, "You are, of course, an honest man, and you say the wallet you lost had $800 in it. Therefore I'm sure it did. But if the man who found this wallet is a liar and a thief, he wouldn't have returned it at all. Which means that this wallet must belong to somebody else. If that man steps forward, he'll get the money. Until then, it belongs to the man who found it."
"What about my money?" the rich man asks.
"Well, we'll just have to wait until somebody finds a wallet with $800 in it..."
HT: John Byron