Yoma 10B

דילמא אתי לאפרושי מן החיוב על הפטור ומן הפטור על החיוב.

In that case, there is the concern lest one come to separate tithes from the obligated produce to fulfill the obligation for the exempt produce, or from the exempt produce to fulfill the obligation for the obligated produce. Produce that one is obligated to tithe by rabbinic law has the status of exempt produce by Torah law. Since it is difficult to distinguish between produce that one is obligated to tithe by Torah law and produce that one is obligated to tithe by rabbinic law, one might seek to fulfill his obligation by separating tithes from one for the other. In both cases, both the produce designated as a tithe and the produce for which it was tithed would retain the status of untithed produce. Therefore, R' Yehuda could not have said that a sukka is considered a house by rabbinic law.

אלא אמר אביי: בשבעה ⁠— דכולי עלמא לא פליגי דמיחייבא. כי פליגי בשאר ימות השנה. רבנן סברי: גזרינן שאר ימות השנה אטו שבעה, ורבי יהודה סבר: לא גזרינן.

Rather, Abaye said: The dispute with regard to the mezuza in the Parhedrin chamber must be explained differently. During the 7 days that the High Priest lives in the Parhedrin chamber during his sequestering, everyone agrees that the chamber is obligated in the mitzva to affix a mezuza there. When they disagree is with regard to the rest of the days of the year, when no one resides there. The Rabbis hold: We issue a decree and require that a mezuza be affixed during the rest of the year due to those 7 days that the High Priest lives there; and R' Yehuda holds: We do not issue that decree, and there is no obligation to affix a mezuza to the chamber the rest of the year.