Muwatta Imam Malik Book of Business Transactions, Hadith 87

Malik said there was no harm if a man who sold some drapery and excluded some garments by their markings, stipulated that he chose the marked ones from that. If he did not stipulate that he would choose from them when he made the exclusion, I think that he is partner in the number of drapery goods which were purchased from him. That is because two garments can be alike in marking and be greatly different in price.
Malik said, "The way of doing things among us is that there is no harm in partnership, transferring responsibility to an agent, and revocation when dealing with food and other things, whether or not possession was taken, when the transaction is with cash, and there is no profit, loss, or deferment of price in it. If profit or loss or deferment of price from one of the two enters any of these transactions, it becomes sale which is made halal by what makes sale halal, and made haram by what makes sale haram, and it is not partnership, transferring responsibility to an agent, or revocation."
Malik spoke about some one who bought drapery goods or slaves, and the sale was concluded, then a man asked him to be his partner and he agreed and the new partner paid the whole price to the seller and then something happened to the goods which removed them from their possession. Malik said, "The new partner takes the price from the original partner and the original partner demands from the seller the whole price unless the original partner stipulated on the new partner during the sale and before the transaction with the seller was completed that the seller was responsible to him. If the transaction has ended and the seller has gone, the pre-condition of the original partner is void, and he has the responsibility."
Malik spoke about a man who asked another man to buy certain goods to share between them, and he wanted the other man to pay for him and he would sell the goods for the other man. Malik said, "That is not good. When he says, 'Pay for me and I will sell it for you,' it becomes a loan which he makes to him in order that he sell it for him and if those goods are destroyed, or pass, the man who paid the price will demand from his partner what he put in for him. This is part of the advance which brings in profit."
Malik said, "If a man buys goods, and they are settled for him, and then a man says to him, 'Share half of these goods with me, and I will sell them all for you,' that is halal, there is no harm in it. The explanation of that is that this is a new sale and he sells him half of the goods provided that he sells the whole lot."