Demai4: 5
הָאוֹמֵר    לְמִי שֶׁאֵינוֹ נֶאֱמָן עַל הַמַּעַשְׂרוֹת: ,,קַח לִי מִמִּי שֶׁהוּא נֶאֱמָן וּמִמִּי שֶׁהוּא מְעַשֵּׂר“ — אֵינוֹ נֶאֱמָן; ,,מֵאִישׁ פְּלוֹנִי“ — הֲרֵי זֶה נֶאֱמָן. הָלַךְ לִקַּח מִמֶּנּוּ, וְאָמַר לוֹ: ,,לֹא מְצָאתִיו, וְלָקַחְתִּי לָךְ מֵאַחֵר שֶׁהוּא נֶאֱמָן“ — אֵינוֹ נֶאֱמָן.
[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.
Demai4: 4
מִי שֶׁקָּרָא שֵׁם לִתְרוּמַת מַעֲשֵׂר שֶׁל דְּמַאי וּלְמַעְשַׂר עָנִי שֶׁל וַדַּאי —לֹא יִטְּלֵם בַּשַּׁבָּת. וְאִם הָיָה כֹהֵן אוֹ עָנִי לְמוּדִים לֶאֱכֹל אֶצְלוֹ — יָבוֹאוּ וְיֹאכְלוּ, וּבִלְבַד שֶׁיּוֹדִיעֵם.
One who designated terumas maaser of demai, or maasar ani of definite [tevel], may not take them on the Sabbath. But if a Kohen or a pauper are accustomed to eat with him, they may come and eat [the tithes], provided that he informs them.