Al-Muwatta Hadith 31.55

Malik said, "Another example of that is that the Messenger of Allah, may Allah bless him and grant him peace, forbade the sale called muzabana and granted an indulgence in the ariya for computing the equivalent in dates. It was distinguished between them that the muzabana-sale was based on shrewdness and trade, and the ariya sale was based on a favour rendered, and there was no shrewdness in it." Malik said, "A man must not buy food for a fourth, a third, or a fraction of a dirham on the basis that he be given that food on credit. There is no harm in a man buying food for a fraction of a dirham on credit and then he gives a dirham and takes goods with what remains of his dirham because he gave the fraction he owed as silver, and took goods to make up the rest of his dirham. There is no harm in that transaction." Malik said, "There is no harm in a man placing a dirham with another man and then taking from him known goods for a fourth, third, or a known fraction. If there was not a known price on the goods and the man said, 'I will take them from you for the price of each day,' this is not halal because there is uncertainty. It might be less one time, and more another time, and they would not part with a known sale." Malik said, "If someone sells some food without measuring precisely and does not exclude any of it from the sale and then it occurs to him to buy some of it, it is not good for him to buy any of it except what it would be permitted for him to exclude from it. That is a third or less. If it is more than a third, it becomes muzabana and is disapproved. He must only purchase from what he would be permitted to exclude, and he is only permitted to exclude a third or less than that. This is the way of doing things in which there is no dispute with us."