Abd al-Rahman bin Abi Layla narrates from one of the Companions (may Allah be pleased with him) that he (peace and blessings be upon him) "forbade going out to meet the trade caravans, and also prohibited a town-dweller from selling on behalf of a Bedouin." "Whoever buys a goat whose milk has been held back has one of two options: if, after milking it, he is pleased with it, he may keep it. If he returns it, he should give a sa' of grain or a sa' of dates along with it."
The Shaykh (may Allah have mercy on him) said: It is possible that this is a doubt on the part of some of the narrators, (meaning) one sa' of this or of that, not a matter of choice, so that it agrees with the established hadiths.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10726