C. F. Keil and Franz Delitzsch on Deuteronomy 15:18

Deu 15:18

If, on the other hand, the servant (or maid) wished to be set free, the master was not to think it hard; “for the double of the wages of a day-labourer he has earned for thee for six years,” i.e., not “twice the time of a day-labourer, so that he had really deserved twice the wages” (Vatablius, Ad. Osiander, J. Gerhard), for it cannot be proved from Isa 16:14, that a day-labourer generally hired himself out for three years; nor yet, “he has been obliged to work much harder than a day-labourer, very often by night as well as day” (Clericus, J. H. Michaelis, Rosenmüller, Baumgarten); but simply, “he has earned and produced so much, that if you had been obliged to keep a day-labourer in his place, it would have cost you twice as much” (Schultz, Knobel).