פירוש על עבודה זרה 43:4

The baraita rules that if the field is already jointly owned by the Jew and the non-Jew, the Jew cannot let the non-Jew take the profits for the work done on Shabbat in return for the Jew taking the profit for work done on Sunday. They can only do this if they make the arrangement at the outset before they join into the partnership agreement. Evidently, Ravina thought that the saffron growers did not make their arrangement before Shabbat.