Overcharged

The Gemorah says that אונאה less than a sixth one doesn’t have to return.  The Gemorah says that the reason one doesn’t have to return the money is because the purchaser is מוחל that minute amount of money.  The Rosh Babba Metziah (Ch.4 siman 20) is unsure whether one is allowed to charge less than a sixth for its not part of the issur or it is forbidden just there is no need to return the money for the purchaser is willing to forgo this small difference in price.  According to the side that it is allowed one must understand the Gemorah that says its permitted because of מחילה is since it is the normal way for people to be מוחל in that situation, it isn’t part of the issur.  This doubt seems to be debated by the Chinuch (#337) on the side that its permitted vs. the Ramban (our parsha 25:15) who holds its prohibited. 

What we do see is that the issur of taking אונאה is not dependent on the obligation to return the overcharged amount.  That raises the question as to why there is no lashes for taking אונאה because its a לאו הניתן לתשלומין, we find the issur even without the need to return the money?  The Even Haezel explains that לאו הניתן לתשלומין is a din in the לאו.  The לאו is defined as a לאו which is a monetary לאו and therefore there is no punishment of lashes attached to it.  

The Torah never mentions explicitly that one must return אונאה that was taken from the buyer.  So how do we know that one must return the additional charge?  The Pnei Yehoshua Bava Metziah 56a explains that the additional charge is considered stolen goods and therefore must be returned.  The difficulty is that according to the Pnei Yehoushua every case of אונאה should become bittul mekach for the stipulation of the sale price is null and void?

Rav Chayim (Mechirah Ch. 15) says that the price tag still stands, and the money is considered to be payment for the object that was purchased; the money that must be returned is a separate obligation that the Torah imposes.  It is still unclear where do we see the Torah imposes such an obligation?