Charles Ellicott Commentary Deuteronomy 15:12

"If thy brother, a Hebrew man, or a Hebrew woman, be sold unto thee, and serve thee six years; then in the seventh year thou shalt let him go free from thee." — Deuteronomy 15:12 (ASV)

In the seventh year. —This is to be understood as the Sabbatical year whenever it came. It would rarely happen that the Hebrew slave would serve for the full period of six years.