פירוש על במדבר 31:28

AND THOU SHALT LEVY A TRIBUTE UNTO THE ETERNAL. The reason for this tribute [given to the priests and Levites] was also because this spoil was taken from [a war constituting] the vengeance of the Eternal¹⁰⁸Above, Verse 3. on a land that was not theirs. But [from the spoil taken] in the lands of Sihon and Og they did not give any part at all to the priests and Levites; [on the contrary], they were even warned against [taking of] it, as it is said, neither shalt thou [Aaron] have ‘any’ portion among them, even in the spoil.¹⁰⁹Ibid., 18:20, according to the explanation of the Sifre, Korach 119.

והרמות מכס לה' “you are to impose a levy (tax) for Hashem.” Nachmanides writes that the reason for this levy is that the loot under discussion was secured from the vengeance due to Hashem. When similar or even larger quantities of loot were secured as a result of the wars with Sichon and Og, Hashem, i.e. the priests and Levites His representatives, did not receive any share of it at all, as the priests and Levites by definition had been excluded from sharing in the lands of the Canaanite nations. They had not been excluded from loot from any other people or tribes.

V. 28. מכם (siehe Schmot 12, 5).