John Calvin 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 unto the Lord. God now requires a tribute, or holy offering, out of the spoil from both parties, but in unequal portions: the people paying ten times more than the soldiers. There was a twofold reason and purpose for this tribute. It was not fair that the Levites alone should be sent away empty, as if their condition were worse than that of the rest, because they were engaged in the service of God and in taking care of the holy things.

But the part which He assigns to them, God commands to be offered to Himself, so that people might not only regard equity among themselves, but that religion might hold the foremost place. For nothing can be more unreasonable than for the rights of people to be maintained inviolate while God Himself is overlooked.

Therefore, to testify to their piety, the offering was prescribed, as if God claimed for Himself the glory of the victory in taking this fiftieth and five-hundredth portion. However, since He needs nothing, having full satisfaction in Himself alone, the Levites are substituted in His place so that they might receive some reward for their ministry.

Again, we perceive that God dealt more generously with the soldiers than with the rest of the multitude; nor is this surprising, for since He had laid a greater burden upon them, it was just that they should be enriched by more fruits of the victory, for He heaps blessings upon blessings according to His pleasure.

From this distribution, we also gather that it depends on His ordinance that some should be richer than others; for, if there were no such thing as property, there would be no test of justice and integrity.