Baba Basra 105a - Grabbing After the Safeik

Tosafos gets into a discussion whether tefi'sa helps even after the safeik is realized to change the muchzak to be the grabber and force the other person to bring a proof. The gemara which says that when one rents a place without it being clear whether the extra month is included in the price, if the renter makes the claim on the first day of the month he must bring a proof since he is trying to be motzi from the owner. But if the renter shows up with his claim on the last day of the month, after already living there that month, he is now a muchzak because the owner is trying to get the money out of him. This implies that the grabbing of the renter changes who the muchzak is. Tosafos rejects this approach and holds that tefisa doesn't work after the safeik has been realized, and the only reason the renter has the upper hand at the end of the month is that the fact that the owner let him live there indicates that he admits to him. B'kitzur, it is a big machlokes rishonim whether tefisah helps to become a muchzak - see rashi kesubos 20a who holds that it does, tosafos argues. see tosafos 2a d.h. l'fichach.

The Shev Shmaitsa (4:16) quotes the shach in takfa kohen (123) who says that one has the right to make a claim of "kim li" - i hold like, even when he is choosing a minority against a majority, and even to grab away from the muchzak. The shach explains that we don't follow majority when it comes to monetary issues, so a person has the right to grab the object from someone else, and then claim "kim li" like the minority view who hold that it belongs to him. The Shmaitsa disagrees and holds that only one who is the true muchzak can claim kim li to prevent the other one from taking it away, but one cannot grab from the other and then use the claim of kim li. R' Shlomo Zalman in his comments on the shev shmaitsa doesn't understand how the shach can claim that a grabber is treated exactly like a muchzak to be able to say kim li like a minority opinion and prevail. His logic is that the whole reason that a muchzak can claim "kim li" like a minority opinion and we don't follow the majority, is that we pasken המוציא מחבירו עליו הראיה and a rov is not a proof. Therefore, the muchzak can keep the object claiming he holds like the minority opinion. This only makes sense if the muchzak is the original muchzak, but why should reuven be able to grab from shimon and then say kim li. Furthermore, he quotes from the kuntres ha'sfeikos (6:9) who explains that kim li is considered a ta'anas ba'ri - definitive claim. R' shlomo zalman asks, how can we allow every am ha'aretz to claim kim li as if he knows that the minority opinion is correct - ואינו יודע בין ימינו לשמאלו להכריע בדבר שגדולי הדורות לא ידעו להכריע. R' Shlomo Zalman suggests that the entire ability to claim kim li should be limited to a case where the muchzak claims that he knows that it is really his, just that he can't be zo'cheh in din, then he can use the claim of kim li like the minority opinion to maintain what he truly knows to be true. But, how can one use kim li without having any knowledge or ability to support the minority opinion that he is relying on? Even if we are slightly more liberal and allow kim li with any muchzak, we certainly cannot allow one to grab away from the other and then use the claim of kim li! It should be incumbent upon the grabber to conduct himself according to the din and not to grab when he knows good and well that he doesn't have the intellectual capacity to be machri'ah like the opinion that rules in his favor.