Concise Quran Commentary
But when they measure or weigh for people they reduce the measurement and weight. This was the practice of the people of Madinah at the time the Prophet (peace be on him) migrated to them.
Tafsir for this Ayah
Surah 83 : Ayah 3
Surah 83 : Ayah 3
وَإِذَا كَالُوهُمْ أَو وَّزَنُوهُمْ يُخْسِرُونَ
but they give less when they measure or weigh for them.