Deuteronomy 15:18

Where a slave determined to have his freedom, the master was to set him free without grudge; for he hath been worth a double hired servant to thee, in serving thee six years ; literally, double the hire of a hireling he hath served thee six years , i . e . he hath saved to thee as much again as it would have cost thee to pay a hired laborer to do the same amount of work.