Related על עירובין 74:3

[If] an am ha'aretz who said to a chaver, "Buy me a bunch of vegetables," [or] "[Buy me] a loaf of bread," Rabbi Yosei says, there is no need [for the chaver] to tithe [what he purchased for the am ha'aretz, see Dem. 6:12]. Rabbi Yehudah says, he needs to tithe. Rabbi Shimon ben Gamaliel says, if he exchanges the money (i.e., he uses his own money rather than the am ha'aretz's money to purchase the vegetables or the bread, see Minchat Yitzchak here), he needs to tithe.