Q: What forms a binding partnership?
A: The Rambam maintains that forming a binding partnership requires an appropriate act of kinyan (act of acquisition) for each of the joint assets. This includes a common commercial practice (situmta), such as a valid contract. An agreement between two people to share their earnings does not form a binding partnership, even if they made a kinyan sudar, since their earnings are davar shelo ba la’olam and not subject to a kinyan (C.M. 176:1-3; see Pischei Teshuvah 201:2).
The Raavad disagrees, since making a kinyan sudar to share their earnings is like committing themselves to work on behalf of each other; the kinyan applies to their bodies (Shach 176:9).
The Mordechai maintains that mutual reliance, even with a verbal agreement alone, forms a binding partnership for the stipulated time. Others maintain that verbal reliance requires them only to share what they earned meanwhile, but they can retract from sharing future earnings (Rema, C.M. 176:3).