A gentile once lent a sum of money to a Jew. They had no written contract, but they swore their agreement beneath a great tree in the countryside, calling on the Holy One and on the tree itself to be their witnesses.
When the time came to repay, the Jew denied ever receiving the money. He claimed there had been no such loan. Without a contract and without human witnesses, the lender had no legal case.
He brought the dispute to Rabbi Hariri.
The rabbi summoned both men and told them to return the next morning. In the meantime, he quietly took the lender aside and whispered something in his ear. The lender nodded and left town.
The next morning the accused debtor came to court. He waited. And waited. The lender did not appear. The rabbi did not conduct the hearing. The hours passed.
Finally the debtor, irritated, asked Rabbi Hariri what was going on.
"I sent him," said the rabbi, "to fetch a branch from the tree under which you made the loan."
The debtor snorted. "Oh, the tree is far. He won't be back before evening."
Rabbi Hariri looked up from his desk. "Pay the money," he said. "The tree has testified."
The debtor turned white. He had just admitted, from his own mouth, that he knew exactly which tree the loan had been made under — even though moments before he had sworn there had been no loan at all. The rabbi had not needed the branch. He had only needed the debtor's memory to slip for a single second (Gaster, Exempla No. 358).
The teaching is small and sharp: a liar's own tongue eventually testifies against him. The rabbi's job is just to wait for the slip.