For the source text click/tap here: Chullin 94
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Our Mishnah taught that a Jewish person may sell a thigh of an animal to a non-Jew, even with the gid hanasheh still inside.
The reason is that a thigh that is intact is easily recognizable as such, and even if the non-Jew tries to sell the thigh to a Jew, the buyer will realize that the gid is still in the limb.
We therefore do not have to suspect that the non-Jew will mislead a Jew by selling him the thigh.
The Gemara detects that the halacha would be that it would be prohibited for a Jew to sell a thigh that is cut to a non-Jew as long as the gid is still inside.
The danger of this is that this thigh might then be sold to a Jew, and the buyer might very well think that the thigh was cut in order to remove the gid, and he will eat from the thigh without removing the gid which is still inside.
The uncomfortable historical complication
There are difficult texts concerning economic relations between Jews and gentiles, particularly passages involving taʿut akum—a gentile's mistake—and discussions of whether one must correct such a mistake. Some medieval authorities distinguish passively benefiting from another person's error from actively creating the error.
That distinction is crucial:
Not correcting someone's independently generated mistake is not necessarily equivalent to actively deceiving him.