Demai, Chapter Four, Mishnah Five
This mishnah deals with a person who sends out an agent to buy produce. Can he trust that the agent will buy from someone who is known to tithe?
1) One who says to another who is not trustworthy in respect of tithes: Buy [produce] for me from one who is trustworthy or from one who gives tithes, [the messenger] may not be trusted.
2) Buy it for me from so-and-so, he is to be trusted.
3) If he went to buy it from him [and then came back] and said: I did not find him, so I bought for you from another man who is trustworthy, he may not be trusted.
Section one: Since the agent is not trustworthy to tithe, he is also not trustworthy to buy from someone who does tithe. The purchaser will have to tithe the produce that he brings back. We should note that this rule would seem to discourage a person from using an agent who is himself not trustworthy with regard to tithes.
Section two: Here, the purchaser tells him to buy from a certain person who is trustworthy with regard to tithes. So while an agent who is not trustworthy with regard to tithes may not be trusted to go out and look for someone who does tithe, he is trusted to listen to his sender when he tells him to buy from a certain person. Also, in this case, the sender could check up on him and find out whether he really did buy from that person. Since the sender can check up on him, the agent is assumed not to lie.
Section three: However, if he comes back and says that he couldnt find the person he was supposed to buy from but that he did nevertheless buy from someone who is trusted with regard to tithes, the messenger is not trusted, just as he was not trusted in section one. Since he cant be checked up on, he might lie.