In Parshas Korach we learned that Kohanim and Levi'im do not receive a portion in Eretz Yisrael; instead, they are supported by terumos and ma'asros. The Rambam (Shmita 13:11) writes that this is only true of Eretz Yisrael proper, but if a Jewish king were to conquer lands beyond the boundaries promised to the Avos, then Kohanim and Levi'im would receive a portion. Ra'avad disagrees and argues that if Kohanim and Levi'im did receive land it would mean that they forfeit teru"m. While the Ra'avad viewed such an exemption as a reductio ad absurdum, the Kesef Mishne comments that the Rambam may indeed have held that there can be Jewish controlled land which is exempt from terumos and ma'asros. As proof, he points to the example of Surya, land conquered by David haMelech which was exempt.
The Parashas Derachim (derush 6) writes that the Ks"M erred in his reading of the Ra'avad. There are two seperate mitzvos involved in terumos and ma'asros: 1) seperating the ter"m from the tevel wheat; 2) giving the kohein the seperated portion. The Ra'avad meant that if Kohanim received a portion of land, they could not collect the tithed portion of the crop -- mitzvah #2 would not apply. But without question the farmer would have to still perform mitzvah #1 to remove the prohibition of eating tevel.
The example of Surya, writes the Parashas Derachim, is a red herring because crops in those lands are exempt from both mitzvah #1 and mitzvah #2, a clear indication that the exemption has nothing to do with Kohanim receiving a portion of those lands. The gemara tells us that Surya was a special case because David conquered those lands out of order (he should have conquered Yerushalayim first) and hence they had an incomplete kedusha.
The Parashas Derachim raises an interesting question: is the seperation of teru"m an independent mitzvah from giving teru"m or are they two parts of the same coin? For those who were not reading three years ago when we first discussed this, maybe a quick review with some new points next post.