John Gill 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 to you when you send him away free
from you
He should not grudge him his liberty, nor what he gives to him when he dismisses him:

for he has been worth a double hired servant to you in serving you
six years ;
since a hired servant a man is obliged to pay him wages for his work, besides his food, whereas a bondservant received no wages.

Aben Ezra remarks, that this proves that a man might not hire himself for more than three years; or however, whereas a hired servant was sometimes hired for so many years, and this is the longest time of any we read of, a servant serving his master six years, his service must be worth double the service of an hired servant, which at most was but three years:

and the Lord your God shall bless you in all you do ;
thus well using your servants, whether menservants or maidservants.