Zevachim 49A

והרי מעשר ⁠– דהוא נפדה, ואילו לקוח בכסף מעשר אינו נפדה ⁠– דתנן: הלקוח בכסף מעשר שנטמא ⁠– יפדה. רבי יהודה אומר: יקבר. נטמא אין, לא נטמא לא!

But there is the case of the second tithe, which may be redeemed, whereas food that is purchased with second-tithe redemption money may not be redeemed. As we learned in a Mishnah (Ma’aser Sheni 10:10): With regard to food that was purchased with second-tithe money and that then became ritually impure, this ritually impure food must be redeemed with money, with which one must purchase other food. R' Yehuda says: The food must be buried. The Talmud infers from the Mishnah: If the food purchased with the second-tithe money became impure, yes, it may be redeemed, but if it did not become impure, it may not be redeemed. If so, according to R' Yehuda, the halakha that food purchased with second-tithe money must be buried if it becomes impure is more stringent than the halakha for second-tithe produce itself, which may be redeemed if it becomes impure.

התם לא אלימא קדושתיה למיתפס פדיוניה.

The Talmud answers: There, the reason one may not redeem food that was purchased with second-tithe money and that then became ritually impure is not that the halakha is more stringent. Rather, there is a different reason. Since this food was merely purchased with second-tithe money, its sanctity is not as strong as the tithe itself and is unable to grasp its redemption money. Its status as second tithe is weak, and cannot be transferred to a third item. Consequently, if it becomes impure, according to R' Yehuda it cannot be redeemed for money but must be buried, as an item that one is prohibited to derive benefit from.