An heave-offering - Render simply an offering, and compare Numbers 18:24. The verb from which the word here rendered “heave-offering” is derived, is rightly translated “levy” in Numbers 31:28.
Matthew Henry
Concise Bible Commentary
Whatever we have, God justly claims a part. Out of the people's share God required one in fifty, but out of the soldiers' share only one in five hundred. The less opportunity we have of honouring God with personal services, the more should we give in money or value.
SDA Bible Commentary (limited)
Volume 1-7
29. Give it unto Eleazar. As a tithe for the maintenance of the priests and the Levites (see ch.18:21, 24, 26).