Weigh, and add some more

Suwayd ibn Qays and Makhramah al-‘Abdi (may Allah be pleased with him) brought some garments from a town called Hajar to sell them in Madīnah. The Prophet (may Allah's peace and blessings be upon them) went to buy lower garments from them. He bargained with them over the price (An-Nasā’i's narration says: "and he bought lower garments from us", without mentioning the bargaining).
In the market, there was a man who had scales to weigh goods in return for a fee. As he was weighing the goods, the Prophet (may Allah's peace and blessings be upon him) said to him: "Weigh, and add some more." He ordered the man to add some more to the side of the scales that held the merchandise so that it would be heavier than the other. This does not mean that the difference in weight would be great and might harm the seller. There should be a slight difference so that the buyer can be certain (and clearly see) that he has taken his full right without any deficiency.