Kommentar zu Bamidbar 18:30

בהרימכם את חלבו ממנו WHEN YE HEAVE THE BEST THEREOF FROM IT — i.e. after you have heaved the תרומת מעשר (the priest’s share in the tithe) from it.

After you set aside. Meaning: This refers to what remains after the setting aside of the finest part and it does not mean “at the time when you set apart.” Thus the letter beis of בהרימכם ("when you set aside") is in the sense of “on account of” meaning afterwards.

V. 30. ונחשב ללוים, da es nicht heißt: לכם, so ist dies ללוים wohl substantivisch zu verstehen: als etwas den Leviten Gehöriges. מעשר ראשון hat weiter keine קדושה ganz so wie תבואת גרן וגו׳, nachdem תרומה ומעשר ausgeschieden ist (siehe zu Wajikra 22, 15) (תבואת גרן וגו׳ ,(ספרי ist das, was dem גרן und יקב bleibt, unterschieden von דגן מן הגרן וגו׳ (V. 27), welches die davon ausgehobene תרומה bezeichnet. Daher:

ונחשב THEN the remainder MAY BE ACCOUNTED by the Levites as profane (of a non-sacred character) in all respects.

What remains. The letter vav of ונחשב ("and it will be considered") comes to add something, meaning that after you have set aside the finest portion then what remains will be considered…

כתבואת גרן AS THE PRODUCE OF THRESHING FLOOR is to the Israelites. [This is not a mere repetition of verse 27 but is stated in order that] you should not say: Since Scripture terms it (the tithe) “Terumah”, — as it is said, (verse 24) “But the tithes of the children of Israel which they heave as a תרומה unto the Lord” — I may conclude that it is all entirely forbidden to the Levites (just as “Terumah” is forbidden to them); Scripture therefore states, “it shall be accounted unto the Levites as the produce of the threshing floor”. How is it with the remainder of the Israelite’s produce after he has set apart the “Terumah”? It is profane (non-holy)! So, too, is the corresponding portion of the Levite’s tithe profane (cf. Sifrei Bamidbar 122).

So that you should not say. Meaning: Rashi himself is explaining this so that one does not ask how one would assume that what remains was forbidden such that a verse was necessary in order to permit it. Rather it was “so that you should not say…”