Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) "forbade a man to sell foodstuff before taking possession of it." Tawus said: I asked Ibn Abbas (may Allah be pleased with them both): How is that? He said: That is dirhams for dirhams, while the foodstuff is still lying where it was.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10675