John Calvin Commentary Deuteronomy 15:18

"It shall not seem hard unto thee, when thou lettest him go free from thee; for to the double of the hire of a hireling hath he served thee six years: and Jehovah thy God will bless thee in all that thou doest." — Deuteronomy 15:18 (ASV)

It shall not seem hard unto thee. I have lately observed how difficult and inconvenient to the Jews was the observance of this law; therefore, it is not without reason that God reproves their mean and stingy pride if they freed their slaves grudgingly.

And, indeed, He first urges them to obey on the grounds of justice, and then from the hope of reward. For He reminds them that for six years the slave had earned double the wages of a hired worker, either because his life was more laborious, since heavier tasks are required from slaves than from free men, who are paid for their work; or because he had completed twice as long a period as hired workers were usually engaged for.

For the Jewish commentators¹⁴⁹ infer from this passage that three years was the term prescribed for hired servants, and thus they suppose the six years were counted. But since this is a mere conjecture, I do not know whether my opinion is not more suitable: that for six years their labors had been twice as profitable as would have been those of a free man who is not under the compulsion of a slave.

¹⁴⁹ “The Chaldee, Vatablus, and other more recent commentators translate it, Since he has served thee for six years for double the wages of a hireling; which the Hebrews thus explain, that the wages of a slave of six years’ standing are called double, because hirelings amongst the Hebrew’s only engaged themselves for three years, whereas the slave served for sir years; therefore he served twice as long, and earned twice as much.” — Corn. a Lapide ; which the Hebrews thus explain, that the wages of a slave of six years’ standing are called double, because hirelings amongst the Hebrew’s only engaged themselves for three years, whereas the slave served for sir years; therefore he served twice as long, and earned twice as much.” — Corn. a Lapide in loco..