Matthew Poole and continuators on Deuteronomy 15:18

18 It shall not seem hard unto thee, when thou sendest him away free from thee: for he hath been worth a double hired servant to thee , in serving thee six years, and the LORD thy God shall bless thee in all that thou doest.

Or, he deserves double wages to an hired servant, because he served thee upon better terms, both without wages, which hired servants require, and for a longer time, even for six years, as it here follows, whereas servants were ordinarily hired but from year to year, Levit. 25. 53. or at most but for three years, as they gather from this place and Isa. 16. 14.

Marginal references: See Isa. 16. 14.