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. (Deuteronomy 15:18)
God is ever ready to bestow rich blessings upon men, but He can do so only for those who appreciate His love enough to love and obey Him (1 John 4:19; John 14:15; 15:10). Should God do otherwise, and bless those who do not serve Him, He would be encouraging them to continue in disobedience.
Worth.
A hired laborer would cost twice as much as a slave.