Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: "Whoever buys an animal whose milk has been held back in its udders, if he wishes to return it, let him return a sa' with it." And he (peace and blessings be upon him) forbade going out to meet the trade caravans.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10727