Bava Kamma 113b:
>The Talmud is not stating that it is permissible to cheat a gentile. We saw above that the Talmud specifically forbids that. What the Talmud is saying is that normal business relations is for each party of a transaction to be responsible for their side of the deal. If Shmuel found a cheap bowl and, after accidentally underpaying, the seller did not count his money, then Shmuel was not obligated to correct the seller.