Demai 4:4-5

Demai 4:4

If a person designated a portion of demai as terumas maaser (for the kohein), or a portion of definitely untithed produce as maaser ani (tithe for the needy), these portions may not be separated for distribution on Shabbos. If he has a kohein or a needy person who regularly eats at his table, he may serve them the terumas maaser or maaser ani, respectively, so long as he informs them that he is feeding them food that technically belongs to them.

Demai 4:5

Let’s say that a certain individual is not reliable when it comes to tithes. If one asks him to purchase produce from a reliable person, or from one who tithes, the unreliable person cannot be relied upon when he says he completed his task properly. If he is told to purchase from a particular individual, he can be relied upon (because this is easily verifiable). If he says, “I couldn’t find that person so I bought produce from someone else who is just as good,” he cannot be relied upon.