army military war - What was the nature of the tax of 32 people?

BeMidbar 31:40 states that, after the war with the Midyanim, in which Bnei Yisrael were instructed to take captives and booty, and to pay taxes to HaShem from this plunder, they "paid" a total of thirty-two (32) people.

What does that mean? Were the captives enslaved? Did they perform work in the Mishkan? What was their status and what became of them?