Deuteronomy 15:18

It must not be a hardship in thy eyes when thou sendest him away from thee; for double the hire of an hireling hath he been of service to thee six years; and on his account the Lord thy God hath blessed thee in all that thou hast done.

you must not begrudge… We find the verse “three years, as the years of a hireling” [Isaiah 16: 14] from which we learn that a man is not permitted to hire himself out for more than three years. This principle applies in equal force to the employer (that is, he who pays his wage).

Thus, the word “twice” denotes “double” (as it does in “there will be twice” [Exodus 16: 5]), when the word serves as a noun. On the other hand, when the same word appears in such phrases as “second to the king” [Esther 10: 3] the word functions as an adjective. It comes from the word “two”, as I have explained [comment on Esther 10: 3]. Scripture thus clarifies here that the phrase “at the seventh year he shall go out free, without paying” [Exodus 21: 2] denotes the beginning of the seventh year. Further proof could be brought from the case of the Sabbatical year, or even from the case of the Sabbath day.

The word will bless you denotes the future, not the past. When a vav precedes a verb beginning with a labial consonant, there is no way to distinguish between the two tenses, so the meaning must be known from context.

לא יקשה בעיניך, when you dismiss her from your service having given him money, etc. משנה, double שכר שכיר, the wages paid to a hired hand, (the Torah knows that you paid him 6 years’ wages in advance, and with what you are giving when he leaves he has in effect received twice what a hired hand would have received.) In recognition of how you have treated him, G’d will bless you in all your undertakings. The meaning of this paragraph is similar to the admonition of the Torah in verse 10 and the use of the term רע בעיניך occurs also in such a sense in Deut. 28,56 when the reaction of even a mother and wife to the pressures of famine are described. רועת עין, describes envy and jealousy in matters financial. The same is true of the expression קשיות הלב, being hard-hearted. Both expressions refer to the feelings of a person when he has to give of his money to others who he does not feel have a claim on his money, on his generosity. Those commentators who understand our verse as speaking of the employer feeling put out at having to release their worker and the Torah telling the employer that he had no cause for the worker had worked diligently for six years are in error. The master knew in advance that after six years the contract was complete and the worker would be free to go. What possible reason could the employer have of being jealous of that arrangement?
Furthermore, the Torah should not have written משנה שכר שכיר, but merely משנה שכיר. Some commentators take the view that the words משנה שכיר refer to the six years, seeing that the average period for which a labourer was hired used to be 3 years, so that the word משנה would refer to the fact that this labourer worked for twice that period. The idea that the standard period was three years is reflected in Isaiah 16,14 This is also senseless, seeing that in Isaiah 21,16 we are told that the average period of hire was precisely a year, no longer. It follows that the words כשני שכיר (as the years of a hired hand) mean precisely 3 years just as when he is hired for a year it means precisely a year.

לא יקשה בעיניך, to furnish him with some of your wealth upon his departure, seeing that he has served you faithfully and you will not become poor by doing so.

כי משנה שכר שכיר עבדך, “for he has worked for you twice as long as the wages that he received,“ a hired hand usually hires himself out for a period of three years. We know this from Isaiah 16,14: בשלש שנים כשני שכיר ונקלה כבוד מואב, “in three years, fixed like the years of a hired labourer, the glory of Moav shall shrink;” it follows that someone who has served his master for six years has actually served twice the length of a hired hand. The reason they hired such labourers for a period of three years was that if they had hired such a person for a term of a year or two years, sometimes the years are longer and sometimes they are shorter, depending it had more than one month of Adar. By making the term three years there could not be a dispute about how many days of labour this included.