Bava Metzia 103A

שכיר בזמנו, נשבע ונוטל.

If a hired laborer claims his wages at the proper time, on the day his wages are due, and the employer claims to have already paid him, the laborer takes an oath that he did not receive his wages and takes his wages. So too, in the case of rent, if the landlord demands payment, and the renter claims to have paid, the landlord should be able to take an oath and then take the payment.

שכיר הוא דרמו רבנן שבועה עליה, משום דבעל הבית טרוד בפועליו. אבל הכא, שוכר מהימן בשבועה.

The Talmud explains: Generally, one takes an oath to exempt oneself from paying, not to collect. And it is specifically in the case of a hired laborer that the rabbis imposed an oath upon him, due to the fact that the employer is busy with his workers and might mistakenly think he paid a particular worker when in fact he had paid a different one. But here, in the case of rent, where there is no such concern, the renter is deemed credible that he already paid the rent, provided that he takes an oath.

אמר רבא אמר רב נחמן: האי מאן דאוגר ליה ביתא לחבריה לעשר שנין, וכתב ליה שטרא, ואמר ליה: נקיטת חמש שנין ⁠– מהימן. אמר ליה רב אחא מדפתי לרבינא: אלא מעתה, אוזפיה מאה זוזי בשטרא, ואמר ליה: פרעתיך פלגא ⁠– הכי נמי דמהימן?

§ Rava says that Rav Naḥman says: In the case of one who rented out a house to another for 10 years and had written him an undated document attesting to that fact, and later the landlord said to the renter: You have already taken 5 years of your rental period, he is deemed credible. The burden of proof is not on the landlord, and the renter cannot use the document to demonstrate that he has a right to rent for 10 more years. Rav Aḥa of Difti said to Ravina: If that is so, then if one lent 100 dinars to another, with a promissory note, and later, the borrower said to him: I have already repaid you half the loan, so too should he be deemed credible? This is not the halakha.