1)    One may not oppress one’s friend financially in any way for even the tiniest amount. What is this “oppression”? Any person who has received someone else’s money in a permissible way (e.g. by borrowing from someone or someone working for him) and does not want to pay him, or he pushes him away by saying “Go and come back later!”. (Kitzur Shulchan Aruch 182)

2)    One who buys an object from a Non-Jew may not trick him regarding the calculation of the money that he will pay. (When the vendor makes a mistake by himself and there is no Chillul Hashem one may ask a Sha’aleh). (ibid)