Deuteronomy 15:18

It shall not be hard in thine eye] See on Deuteronomy 15:9. How well this legislator knew the hearts of his people may be seen from Jeremiah 34:8 ff.

for to the double of the hire of an hireling hath he served thee Jewish commentators inferred from this that the hired servant served only for three years! (Cornelius a Lapide in loco). Calvin thinks that it means that a slave under compulsion worked twice as hard which is contrary to experience. Rather, the cost of keeping a slave was only half of the current wage for a free servant.

and the Lord thy God shall bless thee See Deuteronomy 15:10.