There is a machlokes Rav and Shmuel how the investing partner must compensate the managing partner for the labor that is invested in the iska deal. Rav says that they can set a bar, until 33% profit they will split evenly, but beyond the 33% the managing partner can keep it all. Shmuel says that this doesn't work because profits may never reach 33%, so the investor must ensure at least some money that the managing partner will definitely receive as compensation for his work.
Tosafos asks, how can rav allow this, since the profits may not exceed 33% and Rav doesn't hold like R' Yehuda who is matir tzad echad b'ribis? Tosafos answers that it is very common for the profits to exceed 33% and is therefore permitted. Tosafos seems to be matir based on the fact that the violation of ribbis is only d'rabonon, and since there is a strong chance that it won't be a problem of ribbis it is permitted. Tosafos made a similar point on 68b by the eggs.
Why does Tosafos need this. The fact that there is a chance of profits exceeding 33% places a cash value on that chance. Why isn't the chance also valued as a definite amount that he manager would be receiving since it is in his control to sell that chance to someone else for real money? It should be similar to the gemara in makos where we deal with the zechus s'feiko and zechus s'feika by evaluating what someone would pay for that chance. Here too, someone would pay a real cash value for the chance of winning all profits that exceed 33%?