Demai 6:12-7:1

Demai 6:12

If an unlearned person asked a learned person who was going to the market to pick up a bunch of vegetables or a loaf of bread for him, the learned person doesn’t specify which item he is buying for himself and which is for his unlearned friend. He need not tithe what he gives his unlearned friend (which the learned person would have to do if he gave the unlearned person a gift from his own produce). However, if he specifies when purchasing what’s his and what’s the friend’s, if they get mixed up, he must take tithes before giving it to him, even if the thing that needs tithing is only one in a hundred similar items.

Demai 7:1

Let’s say that person A invites person B for a Shabbos meal but B does not rely on A to take tithes, yet he accepts the invitation anyway so as not to offend. In such a case, before Shabbos, B should stipulate as follows: “What I separate tomorrow will be maaser. Adjoining that will be the rest of the maaser. What I designated maaser will serve as terumas maaser for it. The maaser sheini is to the north – or the south – of that, and I redeem it with money.” (Effectively, he need only separate 1/100 of what he will eat, which will be terumas maaser; the rest of the food is then permitted.)