Sanhedrin 113A

וליפרקיה, דתנן: הלקוח בכסף מעשר שנטמא ⁠– יפדה. כרבי יהודה, דאמר: יקבר. אי הכי, מאי איריא עיר הנדחת? אפילו דעלמא נמי!

The Talmud challenges: And let him redeem that which was purchased with second-tithe money and became ritually impure, as we learned in a Mishnah (Ma’aser Sheni 3:10): An item that is purchased with second-tithe money and that became ritually impure should be redeemed. The Talmud answers: The Mishnah is in accordance with the opinion of R' Yehuda, who says: An item that is purchased with second-tithe money and that became ritually impure should be buried, and it may no longer be redeemed. The Talmud asks: If so, and the Mishnah is referring to an item purchased with second-tithe money that became ritually impure, why did the Mishnah cite this halakha specifically in the case of an idolatrous city? The same would hold true even in cities in general as well, as in R' Yehuda’s opinion, the halakha there too is that the item is buried.

אלא לעולם בטהור, וכגון דנפול מחיצות, וכדרבא דאמר רבא: מחיצה לאכול ⁠– דאורייתא, לקלוט ⁠– דרבנן. וכי גזור רבנן כי אתנהו למחיצות, כי ליתנהו למחיצות ⁠– לא.

Rather, actually, the case in the Mishnah is with regard to pure second-tithe produce of an idolatrous city that was taken into Jerusalem, and it is a case where the walls of Jerusalem then fell. And this halakha is in accordance with the opinion of Rava, as Rava says: The capacity of the wall of Jerusalem to enable one to partake of second-tithe produce is by Torah law. By contrast, the capacity of the wall of Jerusalem to admit second-tithe produce, in the sense that once it enters Jerusalem the produce assumes the status of the property of the Most High and may no longer be redeemed, is by rabbinic law. And the case where the rabbis issue the decree that entry into Jerusalem admits the produce is where the wall is intact; however, in a case where the wall is not intact, no, the rabbis did not issue a decree, and the second tithe remains the spoils of the idolatrous city.