Charles Ellicott Commentary Numbers 31:28

"And levy a tribute unto Jehovah of the men of war that went out to battle: one soul of five hundred, [both] of the persons, and of the oxen, and of the asses, and of the flocks:" — Numbers 31:28 (ASV)

And levy a tribute to the Lord.— Literally, and you shall lift up (or, heave), etc. The portion assigned to the priests, which was taken from the prey that became the warriors' share, and which is described in Numbers 31:29 as the heave offering of the Lord, was one five-hundredth part of the maidens and of the cattle that had become their share.

The maidens were probably employed as slaves, and the cattle used for the maintenance of the priests. Since, then, the entire booty consisted of 32,000 maidens, 675,000 small cattle, 72,000 oxen, and 61,000 asses, the portion that became the share of the 12,000 warriors was 16,000 maidens, 337,500 sheep and goats, 36,000 oxen, and 30,500 asses, of which the priests' portion was 32 maidens, 675 sheep and goats, 72 oxen, and 61 asses.