Demai, Chapter Six, Mishnah Twelve
This mishnah deals with a chaver (a rabbinic associate) who is acting as an agent for an am haaretz. The question is: does the chaver have to tithe that which he buys for the am haaretz?
1) If an am haaretz said to a chaver, Buy me a bundle of vegetables, or: Buy me a loaf of bread, the chaver may buy it without checking [whether it had been tithed], and he is exempt [from tithing it].
2) But if the chaver said: This one I am buying for myself and this one for my friend, and they got mixed up, he must tithe [them both], even if there were one hundred [for the am haaretz.]
Section one: In this case the chaver acts as the agent for the am haaretz and when he buys the produce, it never actually belongs to the chaver. He need not tithe the produce that he gives to the am haaretz nor does he need to check to see if the produce had been tithed because this is not a case of a chaver giving untithed produce to an am haaretz, rather it is treated as if the am haaretz bought it directly for himself.
Section two: However, if he declared that a certain part was for himself and a certain part for the am haaretz and then he mixed up the produce and no longer knew which of it was for the am haaretz and which for himself, he must tithe it all. The problem is that he might have given the am haaretz the portion that was his, and as we have learned throughout the tractate, it is forbidden to give an am haaretz untithed produce because it is known that he will not tithe it. This is true even if he bought one hundred times the amount for the am haaretzin other words, it is true even if tithing will cost him a significant amount of money.