All In The Giving

The Mishna puts Tractate Challah after Maaser Sheni following the order of when the gifts are given to the kohan.  The Rambam introduces the laws of challah in Ch. 5 of the Laws of Bikkurim and other priestly gifts.  Why does the Rambam change from the order of the Mishna?

Rashi (15:20) ד"ה כתרומת גרן says שלא נאמר בה שיעור.  Rashi on the following possuk on the words תתנו לה' תרומה says לפי שלא שמענו שיעור לחלה נאמר תתנו שיהא בה כדי נתינה.  The two Rashi’s seem to be a contradiction, is there a שיעור or not?  The Nodeh B’yehudah volume 2, Yoreh Deah #201 explains that in regard to being matir the bread to be eaten there is no שיעור, but there is another din, a mitzvah to give the challah to the kohan and for that there is a שיעור.  He suggests that the שיעור of a 24^th that the mIshna says for challah is דאור'.  Most assume that is only Rabbinic (see also Malbim on the Rashi and Minchas Asher,) however the point is that we see besides functioning to allow the bread to be eaten, giving the challah to the kohan plays an essential role in the mitzvah.  See more about this by my father shlita here (and links there.)

Based upon this we understand why the Rambam brings the laws of challah together with the other priestly presents.  He is coming to tell us that challah isn’t like teruma where the purpose of the mitzvah is to permit the grain, the giving to the kohan is critical to the mitzvah.  This is reflected in his sefer Hamitzvot as well.  In the mitzvah of Challeh #133 he says: הוא שצונו להפריש חלה מעריסותינו וליתנה לכהן וכו.  He mentions the giving to the kohan as part of the mitzvah.  However, when he brings the mitzvah of terumah #126 he says: הוא שצונו להוציא תרומה גדולה, he makes no mentioning of giving to the kohan (based upon Binyan Av volume 5 #49.)

The Shulchan Aruch Yoreh Deah 331:19 says nowadays when terumah is burnt because of tumah, one need separate only כל שהוא.  The Gra (#35) asks so then why does he rule in 322:4 that one should separate for challah a 48^th even though its going to be burnt, it should be enough with a כל שהוא?  Furthermore, the Rema in 322:5 that rules one need only separate a כל שהוא says the common practice is to separate a כזית.  What is the reasoning for this practice?  Based upon the aforementioned, it is understood that the שיעור of challah is built into the mitzvah.  Its not a separate din merely to satisfy the kohan, but the שיעור נתינה in ingrained into the mitzvah itself.  The Nitziv comments that תתנו לה' תרומה may allude to the amount of a כזית for it implies a minimalistic amount of חשיבות (see also Moadim U’zmanin volume 3 #268 and Tshuvot V’hanhagot volume 1 #672.)