Charles Ellicott Commentary Leviticus 25:40

"As a hired servant, and as a sojourner, he shall be with thee; he shall serve with thee unto the year of jubilee:" — Leviticus 25:40 (ASV)

But as an hired servant. — The master is in all respects to treat him as one who provides his service for wages for a certain time, and will then be his own master again.

Shall serve thee unto the year of jubile. — Nor could he be kept beyond the year of jubilee. This terminated the sale of his services just as it cancelled all the sales of landed property.