Per Deuteronomy 18:6, a Levite may choose his own time of service at the Temple. He can't be drafted or forced to serve. Once there, though, he doesn't necessarily get to choose his own job. This is a lot like surrendering to God. He calls, but he won't force you to answer except under extraordinary circumstances. But once you have committed to serve him, you don't get to dictate the terms of your service. It's all or nothing.
Vs 6-8 imply that a Levite is only entitled to a share in the firstfruits offerings when he is actively engaged in service at the altar. This also suggests that we should materially support those engaged in spiritual ministry in God's Kingdom.
The firstfruits of your grain, of your wine and of your oil, and the first fleece of your sheep, you shall give him.
Deuteronomy 18:4
Grain, wine, oil, and fleece in this verse aren't meant to limit the firstfruits offerings. They are only stand-ins for all the products of human labor and wealth.
There is no functioning Levitical priesthood on earth and I don't know any believing Levites in my local community, so I try to honor this commandment by giving away some of the first harvest from my garden to friends, family, and neighbors.