Qitzur Shulchan Arukh – 66:10

A business agreement does not successfully make [the collection of the profit by the investor] permissible unless it truly is that way — that he took the money to do with it some business. But if one did not put the money to a business need, rather to repay a loan or the like, then the business agreement does not apply, because it’s false.

However, it can be done in this way: Such as if Re’uvein needs money and he has some merchandise even if it’s in a different place, he can sell [the merchandise]  to Shim’on even at a very low price, on the condition that Reuven can choose that if he does not give (the merchandise) to Shim’on, up to a certain day, he will give him instead a certain amount  — so that Reuven will have a reasonable profit). Shim’on then gives Re’uvein the money, and they make a “qinyan sudor” [a formal acceptance of taking ownership, using a scarf or other small item] to seal the agreement. Shimeon, the recipient, gives a part of his garment [serving as the sudar] to Reuven that he should hold it, and by this he acquires Reuven’s merchandise, even if there are no witnesses. Then the merchandise is the responsibility of Shim’on, the buyer.