הלכה על בבא בתרא 176:19

You should measure and weigh generously. [That is,] you should give more than the exact quantity as it is said:⁷Deuteronomy 25: 15. "You must have a full, just measure." What does the word "just" imply? Says the Torah: "Be just by giving him some of your own."⁸Bava Basra 88b. Rashi explains the apparent redundancy in the phrase “a full just measure” (a full measure is by definition, a just one) stating “although you give a full measure, make it tzedek, just and honest, by adding to it a little of your own; weigh generously, tilt the scale in the customer’s favor.”