Targum Yonasan: "Even Sheleimah" refers to weights and "va'Tzedek" to scales; "Eifah Sheleimah" refers to dry measures and "va'Tzedek", to (weighing) jugs.

Bava Basra, 88b: We Darshen from "va'Tzedek" 'Tzadek mi'shel'cha ve'Ten lo!' - When the scales are exactly correct, one should tip them a little in favor of the purchaser. ¹

Bava Basra, 89b: To prohibit the shopkeeper from selling the exact amount where the Minhag is to add a litle extra and the purchaser from adding a little extra where the Minhag is to weigh out the exact amount - even with the consent of both parties. ²

Bava Basra, 89b: To prohibit the shopkeeper from flattening the pile where the Minhag is to add to it and the purchaser from adding to the pile where the Minhag is to flatten it ¹ - and even if the shopkeeper is willing to deduct accordingly from the price and the purchaser is willing to pay more.

Rashi: To teach us that conversely, ¹ if you possess only correct weights and measures, you will have a lot.

Sifri: The Chachamim learn from here that the merchant should clean his measures once every thirty days and the Balabos, once every twelve months.

Yerushalmi Bava Basra, 5:8: Wherever the Torah presents the reward for a Mitzvah, Beis-Din ¹ are not punishable if the people do not adhere to it.