John Gill Commentary Numbers 31:29

"take it of their half, and give it unto Eleazar the priest, for Jehovah`s heave-offering." — Numbers 31:29 (ASV)

And take it of their half
Of the half part of the prey divided to the soldiers:

and give it unto Eleazar the priest for an heave offering of the Lord ;
by way of thanksgiving for the success and victory, God had given them, by means of which so much booty had fallen into their hands.