Rashi on Leviticus 25:40/רש"י על ויקרא כ״ה:מ׳

1כשכיר כתושב BUT AS A HIRED SERVANT AND AS A SOJOURNER HE SHALL BE UNTO THEE] —Employ him on field-labour or on a handicraft; Treat him as you do any hired servant.

2עד שנת היבל [HE SHALL SERVE THEE] UNTIL THE YEAR OF THE JUBILEE — This implies that if the Jubilee happens to come (lit., happens to meet him) before the six years of his servitude are at an end, it frees him.