Tosfos (DH "Ki Ratzu") states that we know the Gemara mentioned two possible explanations for the word "Mechitzah." One is a wall, and one is a division. When the Gemara asks that if they both commit to building a wall it should not help because they can retract their commitment, it knew that according to the opinion that they commit to building a wall it is possible that a Kinyan was made making them legally bound to build their share of the wall, just as a Kinyan can be made for a builder to commit to build a house. However, according to the opinion that Mechitzah in this case means a division which is intangible, and the Gemara does not yet think that the Mechitzah refers to dividing the property, even a Kinyan should not solve anything, as the Kinyan is not on a specific real object (i.e. a wall).

Tosfos (DH "Kinyan") asks that when our Gemara suggested that a Kinyan made a legal commitment for all parties involved to build a wall, the Gemara retorted that this is a Kinyan of mere words. Rashi (DH "Kinyan") explains that the Kinyan of Chalifin referred to by the Gemara is effective when a physical object is being transferred, not when one is merely making a commitment. Tosfos therefore asks that there is a Gemara in Bava Metzia (94a) which does not seem to agree with our Gemara. The Gemara there says that a "Shomer Chinam" -- "person watching an object for free" can make a Kinyan to accept upon himself the strict responsibility of a "Shoel" -- "borrower" by making a Kinyan. Why does that Gemara not ask that a Kinyan does not work for mere commitments?

Tosfos answers that the case in Bava Metzia is a real Kinyan, as he is commiting himself that he will pay for the object if an accident happens to it. This is akin to an extension of fiscal responsibility which is acquired by a Shomer when he accepts to watch the item. Here in Bava Basra, the person is not commiting himself, but rather commiting to have a certain reality of division. This cannot be achieved through a Kinyan.

All the best,

Yaakov Montrose