Sefer HaChinuch, Mitzvah 482

At the root of this mitzvah lies the purpose that we should acquire in ourselves noble, precious, cherished traits of character; and with a precious, noble spirit we will merit to attain good reward, as the beneficent God desires to do good for Israel. It is our splendor and glory that we should have compassion on a person who served us, and we should give him of what we own as an act of loving-kindness, apart from what we stipulated with him to give him as his wages. It is something understandable by the intelligence; there is no need to continue at length about it…Whether the owner was blessed because of him [the laborer] or not, he is duty-bound to give him a liberal bonus; the bonus of a laborer is for himself alone: a creditor of his may not collect from it. The rest of its details are explained in the first chapter of Talmud Bavli Kiddushin.This precept applies to both man and woman, at the time the Temple is extant: for the law of a Hebrew servant is in effect only at the time that the law of yovel (jubilee) is observed, as I wrote earlier. Yet in any event, even today, “let the wise man hear and gain in learning" (Proverbs 1:5) — that if he hired one of the Israelites and the man served him a long time, or even a short time, he is to give him a liberal bonus when he leaves him — as God has blessed him. [Feldheim translation]