Gemara

(Abaye): R. Eliezer holds that it is uncommon that a lone Yisrael would live with a Nochri, for Nochrim are suspected of murder. We decree only when there are two Yisraelim, for this is common;

R. Meir decrees even about a lone Yisrael, for sometimes a Yisrael lives alone with a Nochri.

Bava Metzi'a 108b (Ameimar): If Reuven buys from a Nochri, the law of Bar Metzra (a neighbor has first rights to buy land) does not apply:

If Reuven bought from a Nochri, he can tell the neighbor "I chased a lion from your border"!

If one sold to a Nochri, we excommunicate him until he accepts to pay for any damages that the Nochri will cause.

Rishonim

Rosh (Bava Metzi'a 9:27): If one sold all his property to one buyer, Dina d'Var Metzra does not apply, even if the neighbor wants to buy all the property. It is not common to find one who will buy all the fields at once. Therefore, when he finds such a person, he should not tell the neighbor, lest meanwhile the buyer retract. Chachamim did not enact to benefit the neighbor if it will harm the seller.

Question: Reuven bought a house from Levi on Shimon's border to the south, and a Nochri lives to the north. Shimon protested the sale, because he borders on it. Can Reuven say that Dina d'Var Metzra does not apply, since a Nochri borders on it?

Answer (Terumas ha'Deshen 338): If it is almost Vadai that had the Nochri known that Levi intends to sell, he would have pursued to buy it, perhaps Dina d'Var Metzra does not apply. It does not apply to buying from a Nochri. Rashi (108b DH Arya) explains that this is the greatest good one could do, to chase a lion away from Shimon! Even though Shimon can say 'I did not ask you to chase the lion away! Since the Nochri wanted to sell, also I could have bought it', Reuven can say 'I rushed to buy it, lest due to the delay, the Nochri will reconsider and decide not to sell it, and he would remain next to you.' Perhaps also here, Reuven can tell Shimon 'had I not rushed to buy it, perhaps you would not have agreed with Levi so quickly about the sale, and meanwhile the Nochri would have found out and forced Levi to sell to him. Stam Nochrim are extortionists. Some Nochrim have some laws of Dina d'Var Metzra among their laws. When the lion is already next to Shimon, we say that Reuven's purchase benefited Shimon. I equate this to here, where the lion does not yet border Shimon, just we are concerned lest it come. Since it is almost certain that the Nochri will seek to buy it, and Stam Nochrim are extortionists, this is like Vadai. A likely Safek overrides a Vadai (Pesachim 9a, regarding the chance that a weasel dragged Chametz).

Terumas ha'Deshen: There is another reason why Dina d'Var Metzra does not apply here. The seller told the buyer 'since a Nochri borders me, people are not eager to buy. If I would pursue the neighbors [to see if any want to buy], in the meanwhile, the buyer would reconsider.' This is why we say that Dina d'Var Metzra does not apply one who sold all his fields. The Rosh says so even if the neighbor wants to buy all of them, for it is rare to find one who will buy all of them. He need not inform the neighbors, lest meanwhile, the buyer retract. However, the Rambam and Or Zaru'a connote that this is only if the neighbor wants to buy only one field, for then the seller will lose. If the neighbor wants to buy all of the fields, Dina d'Var Metzra applies. The way Rashi explains one who sold all his fields, surely the same applies in our case. Even the Rosh could agree that here, it is not a sale. People are reluctant to buy only due to concern for a Nochri neighbor. Since there is another Yisrael in the area, it is common for two Yisre'elim to live with a Nochri, like it says in Eruvin. This is even in one Chatzer, and surely if they are merely in the same Mavoy. Even according to the first reason (Reuven's purchase benefited Shimon, for he chases away a lion), it is hard to invent new reasons not explicit in the Gemara or Ge'onim. However, if there is another reason to be Mevatel the neighbor's right to buy, we can join the first reason, even though that reason is not enough by itself. [In this case,] the buyer used to join with the seller, and even now he shares debts with him. Dina d'Var Metzra does not apply to partners.

Poskim

Shulchan Aruch (CM 175:38/39): If one sold [land] to a Nochri, Dina d'Var Metzra does not apply to a Nochri.

Rema: If one bought property that borders a Nochri, some say that the [Yisrael] neighbor has no right to expel the buyer [and buy it himself]. Some say that one may not rely on this by itself, unless there is another reason why the neighbor cannot expel him.

Beis Yosef (DH b'Terumas): Terumas ha'Deshen was reluctant to rely on his reason alone that Dina d'Var Metzra does not apply when there is a Nochri neighbor. I say that it is a proper reason, and one may rely on it by itself. Dina d'Var Metzra is only due to "v'Asisa ha'Yashar veha'Tov", so we annul it for any reason.

Shach (34): The Terumas ha'Deshen joined the reason of being a partner. The Rema wrote 'some other reason', for he holds like the Beis Yosef (Sa'if 60) that partnership itself overrides Dina d'Var Metzra.

Gra (91): We can reject the first reason, for it is not Vadai [that the Nochri would have bought]. We can reject the latter reason, for it is common for two to live with a Nochri.

Ra'anach (125): Why did Chachamim say that [Dina d'Var Metzra does not apply to Nochrim, for] I chased a lion from your border? If it is because Nochrim cause overt damage and there is no salvation from them, this does not apply to Kara'im (Yisre'elim who rejected Oral Torah), especially in the case here, in which a Kara'ite rented the property and not live there, for it was not a neighborhood of Kara'im. If it is because Nochrim do not accept Torah law about liability for damage, the same applies to Kara'im. The Tur says that we excommunicate one who sells to a Nochri, until he accepts all Ones that will come to the neighbor from damages and distancing from damages. It seems that the primary concern is neighborly damage. Some say that the Cherem is only for selling to a Nochri, but not for renting. I.e. a buyer is wont to build and destroy, but a renter is not. Others say that the Cherem is even for a renting. If there are different opinions, the buyer keeps the property. Perhaps the Kara'ite would have sold to a Nochri; the Kara'ite would not accept damage that will result! However, perhaps we say 'I chased a lion from your border' only when he does so at the time of the sale, but not if he merely evaded concern lest a lion come. The Nimukei Yosef (64b DH Arya) says that one who bought an Oni's unstable house that is prone to fall cannot say 'I chased a lion from your border', for at the time of the sale there is the same Safek lest it fall. The Terumas ha'Deshen distinguished between chasing away a crouching lion, and saving from a lion prone to crouch. He said that a Nochri neighbor uproots Dina d'Var Metzra only if the Nochri was almost sure to buy it had the Yisrael not rushed to buy it. A Kara'ite would agree to sell to a Nochri without any pressure. However, here the Yisrael neighbor was already seeking to buy and distance the lion by himself.

Bach (Teshuvah 18): The Terumas ha'Deshen's law is a Chidush. The simple reading of the Gemara is unlike this. Why did the Gemara teach about buying from a Nochri, if Dina d'Var Metzra does not apply even when a Nochri is the neighbor, and it is a Safek lest he buy?! Surely the law is only if it is almost Vadai, e.g. the Nochri was already seeking to buy it, and with another reason, e.g. the buyer was the seller's partner, like the Terumas ha'Deshen said. Do not rely on the Shulchan Aruch, which does not require another reason. Teshuvos Maimoniyos say that Dina d'Var Metzra is almost an Aseh, so we reject it only for a reason explicit in the Gemara. We establish the seller to be Kosher. He would strive not to sell to a Nochri, and risk a Safek Cherem!

Ginas Veradim (6:8): Also Maharikash says that Dina d'Var Metzra does not apply when a neighbor is a Nochri. We are lenient for the buyer when there is a Safek.