Bava Kamma 9A

ויצאו עליה עסיקין; עד שלא החזיק בה ⁠– יכול לחזור בו, משהחזיק בה ⁠– אין יכול לחזור בו.

and claimants [asikin] come forth about the ownership of the land, disputing Reuven’s prior ownership, as long as Shimon has not yet taken possession of the land, he can retract his agreement to the transaction and is not required to pay for the land. Once he has taken possession of the land, he cannot retract his agreement to the transaction.

מאי טעמא? דאמר ליה: חייתא דקטרי סברת וקבלת.

What is the reason that he cannot retract his agreement to the transaction once he has taken possession? As Reuven says to him: The purchase of the land was like purchasing a tied bag [ḥayta] whose contents are unknown and might not have any value. Since you were aware of and accepted that possibility, as you purchased it without a guarantee, you cannot retract your agreement.

מאימת הואי חזקה? מכי דייש אמצרי.

The Talmud asks: From when is it considered that he has taken possession of the property? He takes possession from the time that he walks upon the boundaries of the land to inspect it.

ודוקא שלא באחריות, אבל באחריות ⁠– לא.

The Talmud notes: And this is referring specifically to a case where the field was sold without a guarantee. But where it was sold with a guarantee this is not so, and Shimon can retract his agreement and refuse to pay for the land even after taking possession of it. The reason is that if the claimants succeed with their claims and seize the land, Reuven will then have to return the sum Shimon paid for it. In order to minimize judicial proceedings, the court allows Shimon to claim that there is no point in paying Reuven now only to have his money returned to him later. Therefore, he can retract his agreement to the transaction.