The second Tosphot (R. Izhak) in Bava Batra page 18b

The second Tosphot (R. Izhak) in Bava Batra page 18b says that the original question on Rava from R. Yose holds that the bees also cause damage, and hold that Rava has to say his law also according to R. Yose. so, the gemara asks, "How can we find bees next to the border in the first place in such a way that the owner of the mustard can tell him 'since your bees are next to the border even though they cause damage, I will also put my mustard there even though they also cause damage.'" This cannot be according to Rava who says anything that causes damage has to be three handbreadths away from the border. Then Rav Papa answers, "It is a case of a buyer." That means (to Rabainu Izhak) that the mustard is next to the border and it was there when the other side of the property was sold, and now the owner of the mustard is telling the owner of the bees to go 6 handbreadths away from the border. This is now good to Rava that the mustard is next to the border because of the sale of the property on the other side, but Rava himself is talking about a case where there was no sale and there are just two people in a courtyard that has a division line and each one has to be three handbreadths away from the border. What is unclear to me is why Rabbainu Izhak changes the arrangement from the bees being next to the border to the mustard being there. Why not say that it is a case of a sale, and the owner of the mustard says, "Put your bees three cubits from the border, and I will put my mustard three cubits from the border." I think the maharam fromlublin explains this in his clear explanation of this tophot, however i have not yet been able to understand him. i hope to get to the litvak beit midrash where there is a bava batra and reb aaron kotler' ''chidushei reb aaron'' to help me understand this rabbainu izhak [who is in fact explaining the approach of his grandfather, rashi, who is short in his explanation of this subject.] i might mention here that reb aaron kotler actually says tat rashi and ri migash are almost but not quite identical in this approach. that being the case this means that this approach of rabbanu izhakin our tosphot is very close to the actual halacha in the shulchan aruch of rabbainu yosef karo who follows the rambam who follows his rav, the ri migash. I think the answer to this question might be this: the next question of the gemara is “If it is a case of a buyer, then why do the sages disagree? Now if the bees also do damage, then it is simple that the sages would say once the bees are there already, then the mustard must be kept away (they both cause damage to each other). only now the original assumption has changed, and the bees are not considered to cause damage according the sages and so it makes sense e that the mustard should be kept away from them 6 handbreaths