Demai, Chapter Three, Mishnah Five
This mishnah deals with a person who is at an inn and gives the innkeeper produce for her to cook and prepare for him.
1) One who gives [produce] to an innkeeper [to prepare it for food], he must tithe what he gives to her and what he takes back from her, because she is suspected of exchanging it [with her own produce].
2) Rabbi Yose said: we are not responsible for deceivers. Rather, he must tithe only that which he takes from her.
Section one: The innkeeper is suspected of exchanging the produce that the guest gives her for lesser quality produce that belongs to her. Occasionally, if she is generous, she may even do the opposite, exchanging the lower quality produce that she receives for higher quality stuff. In any case, we must suspect that she doesnt tithe her own produce. Therefore, whatever the guest gives to her he should first tithe, lest she eat the untithed produce herself. According to this opinion, a person is responsible to make sure that he doesnt cause others to transgress. The guest must also tithe anything that he gets from her, under the assumption that she did not already tithe it.
Section two: Rabbi Yose says that we shouldnt be concerned with this woman eating untithed produce because if she eats the produce that he gave her to cook for him, she is a deceiver and we are not responsible to prevent deceivers from sinning. Therefore, he doesnt need to tithe the produce that he gives herif she eats it untithed thats her problem. He does, of course, still have to tithe that which she gives him to eat.