Jamieson, Fausset and Brown on Deuteronomy 15:12

De 15:12-19. Hebrew Servants' Freedom.

12. if thy brother, an Hebrew man, or an Hebrew woman, be sold unto thee--The last extremity of an insolvent debtor, when his house or land was not sufficient to cancel his debt, was to be sold as a slave with his family (Le 25:39; 2Ki 4:1; Ne 5:1-13; Job 24:9; Mt 18:25). The term of servitude could not last beyond six years. They obtained their freedom either after six years from the time of their sale or before the end of the seventh year. At the year of jubilee, such slaves were emancipated even if their six years of service were not completed [see on Le 25:39].