[If] one says to a person who is not trusted regarding tithes, ‘‘Purchase [produce] for me from someone who is trustworthy,’’ or ‘‘from someone who tithes,’’ [the agent] is not believed. [If he says, ‘‘Purchase produce] from So-and-So,’’ [the agent] is believed. [If the agent] went to buy from him, and said to [the one who sent him], ‘‘I did not find him, so I purchased [produce] for you from another [seller] who is trustworthy,’’ he is not believed.