Bava Metzia 35A

איתא לדרב הונא כיון דמשתבע מלוה שאינה ברשותו, היכי מצי מפיק לה? אמר רבא: שיש עדים שנשרפה.

the statement of Rav Huna is so, once the creditor takes an oath that the collateral is not in his possession, how can he produce it after that? Rava said: The Mishnah is referring to a case where there are witnesses that the collateral was burned. Therefore, the creditor need not take an oath that it is not in his possession.

אי הכי, מהיכא מייתי לה? אלא אמר רב יוסף: שיש עדים שנגנבה. סוף סוף מהיכא מייתי לה? דטרח ומייתי לה.

The Talmud asks: If so, the question remains: Why is the obligation to take the oath transferred from the debtor to the creditor? There is no concern that the creditor will produce the collateral. From where will he bring it if it was burned? Rather, Rav Yosef said: The Mishnah is referring to a case where there are witnesses that the collateral was stolen. The Talmud asks: Ultimately, in that case too, from where will the creditor bring the collateral if it was stolen? The Talmud answers: Although the collateral had been stolen, it is possible the creditor will exert himself to locate the thief and bring the collateral, thereby proving that the debtor took a false oath.

אי הכי, כי משתבע מלוה נמי, לטרח לוה וליתי! בשלמא מלוה ידע מאן קא עייל ונפק בביתיה, ואזיל וטרח ומייתי לה. אלא לוה, מי ידע מאן עייל ונפיק בביתיה דמלוה?

The Talmud asks: If so, in a case where the creditor takes an oath as well, let the debtor exert himself and bring the collateral, thereby proving that the creditor took a false oath. The Talmud answers: This is unlikely. Granted, there is concern that the creditor will recover the stolen collateral, as he knows who enters and exits his house, so he may have some inkling of the identity of the thief. And therefore, he goes and exerts himself and brings the collateral. But with regard to the debtor, does he know who enters and exits the creditor’s house? He has no inkling who the thief might be.