ohna'ah

The Pasuk in Parshas Behar says
וְכִֽי־תִמְכְּר֤וּ מִמְכָּר֙ לַֽעֲמִיתֶ֔ךָ א֥וֹ קָנֹ֖ה מִיַּ֣ד עֲמִיתֶ֑ךָ אַל־תּוֹנ֖וּ אִ֥ישׁ אֶת־אָחִֽיו

בְּמִסְפַּ֤ר שָׁנִים֙ אַחַ֣ר הַיּוֹבֵ֔ל תִּקְנֶ֖ה מֵאֵ֣ת עֲמִיתֶ֑ךָ בְּמִסְפַּ֥ר שְׁנֵֽי־תְבוּאֹ֖ת יִמְכָּר־לָֽךְ לְפִ֣י | רֹ֣ב הַשָּׁנִ֗ים 
תַּרְבֶּה֙ מִקְנָת֔וֹ וּלְפִי֙ מְעֹ֣ט הַשָּׁנִ֔ים תַּמְעִ֖יט מִקְנָת֑וֹכִּ֚י מִסְפַּ֣ר תְּבוּאֹ֔ת ה֥וּא מֹכֵ֖ר לָֽךְ

וְלֹ֤א תוֹנוּ֙ אִ֣ישׁ אֶת־עֲמִית֔וֹ וְיָרֵ֖אתָ מֵֽאֱלֹהֶ֑יךָ כִּ֛י אֲנִ֥י יְהֹוָ֖ה אֱלֹֽהֵיכֶֽם

The pasuk is telling a person that he must not cheat on the sale. The topic of this specific sale is land during the Yovel period and the value of the land is dependent on the amount of years left until the next Yovel. The Pasuk is telling us that you cannot cheat on the price based on the years left to Yovel. This seems like a pretty straightforward issue. Why does the Pasuk say "יָרֵ֖אתָ מֵֽאֱלֹהֶ֑יךָ" Those words, Rashi says, are used when there is a need for a person to have moral fortitude, where we cannot simply rely on Bais Din, usually in a situation where the Issur is dependent on the perpetrators intent. 

The word "תּוֹנ֖וּ" is the word "Onah'ah" which is derived from the shoresh "Ohn". When Binyamin is born, Rachel calls him "Ben Oni" which Unkelos translates as "Bar Davai" meaning "Son of My Pain". Ohn= Pain. However there is another definition for Ohn, and that is "Strength" as we see in Birkat Yaakov where Yaakov calls Reuvain כֹּחִ֖י וְרֵאשִׁ֣ית אוֹנִ֑י Here it means "Strength".

It is interesting that the Torah chose to call Ohna'ah by this word. 

A possible explanation is that the issur is to cause pain utilizing strength and power. A normal issur such as Gezel-stealing doesn't require "וְיָרֵ֖אתָ מֵֽאֱלֹהֶ֑יךָ" because there is a system in place to punish those who steal or take other people's property illegally. However, many of the worst people are the people who make the laws or at least people who have enough power to skirt the law. The issur of Onah'ah is directed to them. You can utilize your power to declare something that is immoral as legal (moral and legal are not only not synonymous but also many times mutually exclusive). However you will still be called out by the Torah as being over on the issur of Onah'ah. The reason the Torah uses the term "Onahah' because it is a combination of somebody using his power to cause someone else pain. When the use of power is used to cause pain there is no legal system that can block him because in many instances the perpetrator is actually using the legal system to cause the pain therefore the Torah has to say " וְיָרֵ֖אתָ מֵֽאֱלֹהֶ֑יךָ"

(The big guys have compliance departments, they have lawyers, they have lobbysts, they have all kinds of entities to make sure that whatever they are doing will be legal. Their lawyers are in charge of getting them from point A to point B legally. The little guy on the other hand has no choice but to break the law just to feed his family as he cannot afford the protections of the powerful. Then the powerful come and take down the little guy for competing against them using the excuse that he is breaking the law this has many variations but this can be called the quintessential Ohnah'ah)