Ketubot, Chapter Thirteen, Mishnah Nine
This is the last teaching of Admon. It concerns two people who each have a document of debt from the other.
If two men produced debt documents against one another, Admon says; [the holder of the later document can say to the other,] Had I owed you [any money] how is it that you borrowed from me?
But the Sages say: This one collects his debt and this one collects his debt.
In this case Reuven has a document saying that Shimon borrowed from him, and Shimon has a later document that says that Reuven borrowed from him. Shimon claims that he already paid Reuven back, but that he lost his receipt.
According to Admon, Shimon says that if he really owed money to Reuven, how come Reuven did not just collect his debt, and instead borrowed money from him. Since, according to Admon, this is not logical, it must be that Shimon did not owe Reuven money when Reuven borrowed. Therefore, Shimon collects his debt and Reuven does not collect his.
The Sages say that nevertheless, since both men have documents of debt, they can both collect.