If a non- Jewish cashier forgot to scan an item, do I have to go back and make sure they charge me for it?
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The Avnei Yoshfei has a teshuvah on this subject.
Further business halacha.com says that
Unless the amount is de minimus (of minor importance), then it is an opportunity to make a Kiddush Hashem. Passing over that opportunity is throwing away a Mitzvah, and if caught could lead to a very bad Aveirah. I'm very skeptical that you could find a heter for that. This question discusses it.
That is really the distinction between a Jew and a non-Jew here - for a Jew, even if the amount isn't enough to make a big deal about and the person wouldn't care very much, you have to go to the effort of returning it. For a non-Jew, it more depends on their perception of the situation. A lot of discussion in these Halachos focus on the details, which are important, but tend to elide over the common sense points that apply generally because they are taken for granted and are not tied to the particular facts of this situation.