The gemara understands that if one imposes a neder on himself that forbids him from receiving benefit from his partner, it is legitimate to force him to sell because of a combination of 2 reasons: 1. he is penalized for making the vow, and no one forced it on him. 2. it will prevent him from violating an issur because he is likely to use the property when he sees his partner using it. But, in a situation where one imposes an issur hana'ah on the other, there is no penalty. The reason is that although the first reason applies to the madir, the second does not, and although the second reason applies to the mudar the first reason doesn't.
The question is that according to the rambam that when the mudar violates the neder, the madir is in violation of bal ya'chel, both reasons should actually apply to a madir. He should be penalized for making the vow, and by forcing him to sell it will prevent him from a violation every time the mudar uses the property? The Ran should have used this gemara as his third proof against the Rambam?