Yesterday we learned the Parashas Derachim's chiddush that there are two aspects to the mitzvah of terumos and ma'asros: 1) a mitzvah of hafrasha, seperating a portion to remove the issur of tevel; 2) a mitzvah of nesina, giving the seperated portion to a kohein. The truth is that I had in mind to discuss this idea last week in the context of the mitzvah of challah. The shiur of challah which must be seperated and given to a kohein is 1/24 of the dough for a regular person and 1/48 for a baker. This amount was set by Chazal -- what is the minimum requirement on a d'oraysa level? The Noda b'Yehudah (Mh"T 201, see Minchas Chinuch 385:12) is mechadesh that the minimum shiur depends on whether we are speaking of the mitzvah of hafrasha or the mitzvah of nesina. To remove the prohibition of tevel seperating even the smallest piece of dough suffices; however, to fulfill the mitzvah of nesina requires giving the kohein an amount equal to 1/24 of an isaron (no matter how much dough is kneaded).
Achronim (see Shu"T Shiurei Sheivet haLevi, and apologies because I do not have the sefer at home and I did not jot down the siman) suggest that this chiddush of the Noda b'Yehudah may depend on a machlokes between the Rambam and Ramban. The Rambam counts hafrasha and nesina together as one mitzvah (aseh 133). However, Ramban, disagrees and writes that they should be counted as two distinct mitzvos (see shoresh 12 of Sefer haMitzvos). One of Ramban's many proofs is the fact that we recite a bracha on hafrasha, not on nesina -- if nesina is the culmination of the mitzvah, why would we not recite the bracha at that point?
The Parashas Derachim's reading of the Ra'avad fits nicely with the Ramban's approach -- since kohanim and levi'im can receive a portion of conquered lands, it removes the requirement to give them terumos and ma'asros but does not excuse the mitzvah of hafrasha. At the same time, perhaps the Rambam fits l'shitaso as well. Since hafrasha and nesina go hand in hand, one cannot dismiss one without dismissing the other (though by the same token, since they go hand in hand if hafrasha cannot be dismissed perhaps neither should nesina be, which may be what Ra'avad is driving at).