Why Must One Return אונאה

How do we know that one must return the value of the overcharged אונאה, the Torah only tells us that there is an issur but how do we know the one that cheated must give back what the difference? The Pnei Yehoshua (Bava Metzia 56a) says that the source is the mitzvah to return גזילה.  How could the Pnei Yehoshua say that אונאה is mere theft if the Gemorah Bava Metzia (61a) explains why we can’t derive the issur of אונאה from גזילה?  What the P.Y. means is that we could not learn out the issur of אונאה from גזילה but after the Torah tells us the issur it becomes an extension of the issur גזילה.

In this approach of the P.Y., Rav Chayim (stencil בענין הפקעת הלואת עכו"ם) has two ways to understand it.  One way is the simple approach, the act of אונאה is an act of גזילה, it is an evil act which obligates the returning of the money.  The other approach he suggests is that the actual taking of the money done in the normal parameters of business is not an act of stealing but the issur the Torah places on the money makes the money a cheftzah of tainted money which is in included in the mitzvah to return stolen money.  The difference between these approaches would be if one took charged אונאה to someone that there is no issur to mischarge them such as a gentile is there still an obligation to give back the אונאה.  According to the first approach that the act of אונאה is an act of גזילה, that would be the same even is one mischarges a gentile and one would have to give back the difference.  But in the second approach, that the issur creates the גזילה, since there is no issur to charge אונאה to a gentile, there will be no obligation to return the אונאה.  (Rav Chayim is מכריע like the second way from the Geomrah, the proof is צ"ע, עיי"ש.)

The Tur is Choshen Mishpat siman 227, at the beginning of the laws of אונאה says there is מלקות for violating the issur of אונאה just like there is no מלקות for violating לא תגזול.  This would bolster the opinion of the P.Y.  However, the Ramban in this week’s parsha disagrees.  He bases on the structure of the pessukim that chazal derived from there the law that one must return what they mischarged.  "ודרשו חכמים מפני שאמר הכתוב: וכי תמכרו ממכר לעמיתך או קנה דבר הנקנה מיד ליד, אל תונו איש את אחיו – למדנו שיש באונאה דין מיוחד במטלטלים שאינו נוהג בקרקעות, והוא חזרת הממון, אבל אזהרת הלאו נוהגת בכולן."  It is clear that he holds אונאה does not fall in the category of stealing and that's why one must return it but it is a separate derasha.  Following this line of thought he holds the lav can exist even when there is no need to return the אונאה and holds that applies to land.  According to both approaches of Rav Chayim if there is a lav of אונאה on land the perpetrator should have to return what s/he took. 

 
Rav Chayim (Ch. 15 Laws of Sale) asks a basic question on the entire din of אונאה.  He asks if the cheated party asks for the difference in value back, the price stipulated at the time of the sale has been nullified and then the whole sale should be canceled?  For example, if one sells a $5 for $6 the agreed terms of the sale were to but the object for $6.  Then the buyer comes back with a claim of אונאה and takes a dollar back.  Now the the stipulated price for the sale, the terms of agreement have been ripped up and the seller never agreed to sell his object for only $5, do the whole sale should be voided?  Rav Chayim answers that the din to return אונאה does not affect the terms of the sale.  In other words, the sale goes through for $6 but now there is a new obligation of the seller give $1 to the buyer.  (This is similar to how some understand רבית, it is built into the terms of the loan and there is an obligation on the borrower to pay it but then there is a separate obligation on the lender to return the רבית.)
This Rav Chayim for sure flies in the face of the first approach of Rav Chayim mentioned before.  If taking אונאה is an act of theft how can it be incorporated as part of a legal sale?  The only question is can it fit with his second approach.  Can you say that it is a valid sale but becomes tainted money nonetheless or are the two mutually exclusive, if its a legal sale the money is not tainted?  That is something to think about.