Albert Barnes 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)

A heave-offering — translate simply as an offering, and compare (Numbers 18:24). The verb from which the word here translated “heave-offering” is derived, is rightly translated “levy” in (Numbers 31:28).