[The Ruling on Overreaching the Unsuspecting Buyer When He Is an Oppressor]

The answer—and Allah is the granter of success—is that overreaching the unsuspecting is ribā, as in certain reports. One may also adduce Allah’s saying: “Do not consume one another’s wealth unjustly, except that it be trade by mutual consent among you …” [An-Nisā’:29]. On that basis, it is not permissible to overreach the unsuspecting—absolutely—whether he is an oppressor or not.
There is a situation in which it may be permissible: namely, if this oppressor holds rights that belong to the seller; then it is permissible for the seller to take from the oppressor an amount equivalent to those rights.
The evidence for that is Allah’s saying, Exalted is He: “So whoever has transgressed against you—then transgress against him in a like manner as he transgressed against you.” [Al-Baqarah:194], and His saying: “The recompense of an evil deed is an evil one like it …” [Yūnus:27], and similar passages are many in the Qur’an.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2