Qitzur Shulchan Arukh – 66:3
This se’if is another variant on the case where an investor gives money for someone else to produce profit that they then split. So far we saw that if the worker stands to lose if the deal takes a net loss, he is holding that percentage of the capital for himself, and therefore it qualifies as a loan. Then, his share of the profits would qualify as interest on that loan, and prohibited. However, if he is paid for his work, then he is working for pay, and the deal is permitted.
They can also make a condition that the choice is recipient’s — that if he wants to give to the investor a such-and-such in exchange for his share of the profits, he can do this, and the rest of the profits will remain his. This way is the right one, because presumably that the recipient will not want to take an oath [given the negative view halakhah has of unnecessary oaths even if true], and will give to the investor what was agreed between them. This is [the mechanics of] the “heser iska” that we normally use. And if the recipient knows himself afterward that he did not profit or even that he took a loss, he can give to the investor the principal with the [fraction of the] profit that they agreed on together. There is prohibition in this case, because he has an obligation to take an oath, he can release himself by giving his money as per the oath.
The next halakhah provides a contrasting case.