סִימָן קפג – הִלְכוֹת נִזְקֵי מָמוֹן
183: Laws of Fiscal Damages
It is prohibited to cause financial damage to his friend, even with the intent to repay, just as it is prohibited to rob or to steal with intent to repay. Even causing damage to his friend, whether by act or by speech, is prohibited. For example: Re’uvein who sells merchandise to a non-Jew. Comes Shim’on and says to him [the non-Jew] that it is not worth so much — even though it is true, it is prohibited, for this overpayment is permitted. [Unlike closing a deal in which a Jew willingly overpays, which is not.] Whomever causes damages to his friend, even in a matter which is not punishable by human law, he is obligated by the law of [the One in] heave, until he appeases his friend.
In the the Qitzur’s example, the Jew’s actions qualify Re’uvein as what the Rambam would call “naval bireshus haTorah — disgusting with the permission of the Torah.” The deal is morally wrong, but not so much so as to be the subject of a Torah prohibition which would apply to all people at all times. Therefore, undermining the deal would be fiscal damages, even though the deal is not, deep down, even in Re’uvein’s spiritual best interest. The correct response would be to get the Jew not to take advantage of the non-Jew’s ignorance or naivite. Not to work against him.