Demai 3:6-4:1

Demai 3:6

If one gives his mother-in-law ingredients to prepare for him, he must tithe both what he gives her and what he receives back. This is because we suspect that she may exchange his ingredients for other produce that she prefers. Rabbi Yehuda says this is because she wants to make a good impression. Rabbi Yehuda agrees that if one gives his mother-in-law permitted (ownerless) Sabbatical produce, we do not suspect that she will exchange it for other produce and feed it to her daughter.

Demai 4:1

If one bought produce from a person who cannot be relied upon to tithe it, and then he forgot to tithe it himself before Shabbos (and it cannot be tithed on Shabbos), if he asks the seller on Shabbos, he can rely on his word; once Shabbos ends, he may no longer eat of it until he tithes it. If he couldn’t find the seller to ask him but a reliable person said that he knows this produce to have been tithed, he may rely upon him; once Shabbos ends, he may no longer eat of it until he tithes it. If terumas maaser (which can only be eaten by a kohein) taken from demai got mixed up with regular produce, it would normally render the entire mixture limited to kohanim. In such a case, Rabbi Shimon Shezuri says that one may rely upon the seller’s word even on a weekday.