Responsa for Bava Metzia 31:15

Q. A, B and C live in the same house. A Gentile came to the house and sold to A and B an article at a bargain price, on which they (A and B) made one mark profit. C claims that since part of the house belongs to him, and since he was present at the time the article was bought from the Gentile and he was one of the buyers, he is entitled to one-third of the profits. A and B deny C's claim that he was present at the time the article was bought; they admit that they once bought an article in partnership with C, but say that on that purchase they earned no profit.
A. If A, B and C had made an agreement that they be partners in all good business transactions that come to the house, C is entitled to his share even though he was not present at the time the article was bought; for, although a person cannot sell to another anything that is not yet in existence, people may enter into a partnership to divide future gains (not yet in existence) as in such a partnership each partner merely becomes a hired worker to work for the benefit of the other partners. But if no such agreement exists, A and B must take an oath that C was not one of the buyers of the article, and that he was not present at the time the transaction took place and did not say: "I want to be a partner to this transaction." If, however, C was present at the time and expressed his desire to be a partner, he is entitled to his share of the profits; for, if one person tells another to lift up for him an article which has no owner, and this person picks it up, the former gains title to it; especially so if the second person is also to gain part ownership in the article, as in the present case.
SOURCES: Pr. 325.