Ketubot, Chapter Thirteen, Mishnah Eight
In the scenario in this mishnah, Reuven takes out a document that says that Shimon owes him money. Shimon claims that he paid back the debt, but that he lost his receipt. The mishnah then discusses a possible clue that Shimon might bring to prove that he already paid Reuven back. Again, Admon and the Sages disagree.
If a man produced a debt document against another, and the latter produced [a deed of sale showing] that the former had sold him a field, Admon ruled: [The other] can say, had I owed you [anything] you should have been paid pack when you sold me the field.
But the Sages say: This [seller] was clever, since he may have sold him the land in order to be able to take it from him as a pledge.
In this case, Reuven takes out a document which states that Shimon owes him money. In response, Shimon takes out a sale document which shows that Reuven sold him a field. Shimon says the fact that Reuven sold him a field and collected money from him, proves that Reuven didnt believe that Shimon still owed him money. Had Reuven thought that Shimon owed him money, he should have taken out the debt document then and taken the money and not given him the field. Admon rules that Shimons words are accepted and that he does not have to repay the debt.
However, the Sages rule that Reuven was clever. He may have sold Shimon the land so that later if Shimon defaulted on the debt, Reuven would be able to collect the land. In other words, the fact that Reuven sold him the land does not mean that Reuven didn’t think that Shimon owed him money.