Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Do not sell over one another's sale."
Imam al-Shafi'i (may Allah have mercy on him) says: A man bought goods, and the two had not yet separated when another man came and offered him goods like it, or better than it, for a lower price, so he cancels his companion's sale, since he has the option before they separate. This is corruption, and he has disobeyed Allah if he knew the hadith, and the sale will be binding.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10892