Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "A Muslim should not bargain over the bargain of a Muslim."
(b) Shu'bah narrates from al-Ala' that he forbade a man to bargain over the bargain of his brother, and some say: that a man should sell over the sale of his brother.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10896