[Reuven and Shimon owned a house jointly. Reuven was contemplating selling his share to someone with whom Shimon got along poorly. Shimon asked Levi to help finance Shimon’s buying of Reuven’s share, in return for Levi receiving certain specific rooms in the house. In the meantime, Reuven sold his share to the other person. Shimon wants to use the rights of a bar metzra (neighbor) to make the buyer transfer to him the property at sales price.]

The main reason for the halacha of bar metzra is to allow the neighbor to maximize the potential of his property, by making use of the proximity or expand boundaries. It is therefore not clear that this rule should apply if the neighbor is planning to sell it to someone else. [Ed. note – this idea is slightly surprising, given that halacha recognizes the importance of having neighbors with whom one can get along.] On the other hand, Shimon still has elements of bar metzra, as Reuven and Shimon had only theoretical rights in half the property and now Levi will get specific rooms. This leaves Shimon to receive full control over other rooms, from which he will receive benefit as a result of the law of bar metzra.
Assuming we have a doubt whether the rules of bar metzra apply in this case, we say that the sale to the other person was certainly halachically valid, and there is a question whether the Rabbis’ special takana (Rabbinic institution) to require going beyond the letter of the law applies. In that case of doubt, we should not apply the takana. The Rambam (Gezeila 4:7) takes that approach regarding a different takana, and it seems to be all the more clear in a case in which the bar metzra is trying to extract the property from a buyer who has muchzakut (status quo ownership) over it.
There is another option that Shimon is contemplating using to finance the purchase – to borrow money from a relative to buy Reuven’s rights, in such a way that the lender will live for a period of time in the house in exchange for a reduction in the principal due. On one hand, initially, the arrangement between Shimon and the relative is of a loan, and it is Shimon who seeks to actually buy Reuven’s share. On the other hand, it is likely that Shimon plans to eventually sell the property to the lender, in which case the takana should not apply. Therefore the buyer can make Shimon swear that he does not plan to sell it to the relative. Again, in the case of doubt, the buyer has the benefit of the doubt.
One can counter that Shimon should have more than usual rights of bar metzra, as a partner is more connected to the other part of the rights to the property than a simple neighbor is. This might compensate for some of the weaknesses mentioned in the claim of bar metzra. Still, though, it seems that the reasons to not apply the rules of bar metzra apply even when the bar metzra is a partner. Therefore, Shimon cannot force the buyer to sell him Reuven’s sold rights.