Is Chalipin A Sale

 וכי תמכרו ממכר לעמיתך או קנה מיד עמיתך אל תונו איש את אחיו

The Rif (teshuva cited in Shittah Mikubetzes Bava Metzia 47b) says since it says מקח, therefore חליפין, which is not a מקח is excluded from the parsha of אונאה.  Tosfos Kiddushin (3a) brings the opinion of Rabbenu Tam that the kinyan of chalipin applies to a gentile as well.  Tosfos asks that the Gemorah Bechoros (13a) says או קנה מיד עמיתך מיד עמיתך הוא דבמשיכה הא מיד עובד כוכבים בכסף, a Jew acquires movable goods through physically moving the object, but a gentile's kinyan is with money.  The Gemorah asks maybe they need both money and a משיכה?  The Gemorah answers, דומיא דעמיתך מה עמיתך בחדא אף עובד כוכבים נמי בחדא.  Asks Tosfos, according to Rabbenu Tam there isn't just one method for a gentile to be koneh, there's two, money and chalipin (see there what he answers)?  [I believe Rabbenu Tam would tell you chalipin is in the category of kesef; it isn't counted as its own kinyan, see Sefer Hayashar # 142, however Tosfos understands Rabbenu Tam that at the end of the day chalipin isn't מטעם כסף.]   Says the Ketzos (195:1) according to the Rif it's not difficult for chalipin isn't referenced to in the possuk for its not a מקח.

There are two types of chalipin.  There is the chalipin mentioned in the Book of Ruth, שלף איש נעלו, known as קנין סודר where the kinyan is symbolic of their arrangement, where one exchanges a handkerchief for the object being acquired.  A second type of chalipin is that of שוה בשוה, where two parties exchange objects.  The Ketzos seems to have understand the Rif's opinion is that neither of them is considered a מקח and is exempt from אונאה.  Many Roshei Yeshiva assume that the Rif only said his din regarding the second type of chalipin, קנין שוה בשוה, when there is no object be bought more than the other, its an exchange, a barter, that is not a מקח!  But in the case of קנין סודר, that is a קנין to uphold the sale of an object.  That is a sale, a מקח and would be subject to אונאה.  According to this, the חליפין of קנין סודר would have אונאה and would be included in the possuk.  The basis for this distinction comes from Rabbenu Tam in Bava Metzia (46b and 47a.)  Rabbenu Tam (with added explanation,) understands that the chiddush of chalipin is that the transfer of a סודר constitutes a קנין.  The exchange of שוה בשוה doesn't need to be learnt from chalipin, its understood you can do a barter.  Although Rabbenu Tam takes it to an extreme to assume the two types of chalipin are completely distinct and שוה בשוה doesn't have the general rules of chalipin and other Rishonim don't take it as far, the yesod is the same.  The שוה בשוה קנין is an exchange, not a sale for payment; that is not a מקח.  The chalipin of סודר is a way of making the קנין, but the sale is for the money that will be received; that is a sale and will be subject to אונאה.

Rabbenu Tam follows his means of distinction is Kiddushin (3a) differentiating between חליפין that is worth a פרוטה vs. what's worth less (see Sefer Hayashar #142.)  However, Rabbenu Tam learns that when it is a perutah then its מטעם כסף, when less, its not מטעם כסף (and in Sefer Hayashar that stands למסקנה not like Tosfos in Kiddushin cites him,) which seems to be the opposite of  how the Roshei Yeshiva learn in the Rif that when it is שוה בשוה it is not a מקח, but when its קנין סודר then it is?  ויש לפלפל, I leave to those learning B.M. or Kiddushin.