Halacha

הלכה א
הַלוֹקֵחַ מִן הַסִּיטוֹן וִחָזַר וְלָקַח מִמֶּנּוּ פַּעַם שְׁנִיָּה לֹא יְעַשֵּׂר מִזֶּה עַל זֶה אַף עַל פִּי שֶׁהוּא מִין אֶחָד אֲפִלּוּ מִקֻּפָּה אַחַת. אַף עַל פִּי שֶׁמַּכִּיר אֶת הֶחָבִית שֶׁהִיא הִיא. מִפְּנֵי שֶׁהַסִּיטוֹן לוֹקֵחַ מֵאֲנָשִׁים הַרְבֵּה וּמוֹכֵר. וְשֶׁמָּא לָקַח זֶה שֶׁמָּכַר תְּחִלָּה מֵעַם הָאָרֶץ שֶׁפֵּרוֹתָיו דְּמַאי וְזֶה שֶׁלָּקַח בַּסּוֹף מֵחָבֵר שֶׁפֵּרוֹתָיו מְתֻקָּנִין. וּכְבָר בֵּאַרְנוּ שֶׁאֵין מְעַשְּׂרִין מִן הַחַיָּב עַל הַפָּטוּר וְלֹא מִן הַפָּטוּר עַל הַחַיָּב. וְאִם אָמַר הַסִּיטוֹן מִשֶּׁל אֶחָד הֵן נֶאֱמָן:
כסף משנה
1.
When a person purchases [produce] from a wholesaler and then purchases [produce] from him a second time, he should not separate tithes from one batch for another. [This applies] even if they are of one species, and even from one container, and even if he himself recognized the barrel [from which the produce was taken].1And thus one might think that both batches came from the same person. [The rationale is that] a wholesaler purchases from many different people and sells [it]. Perhaps the produce he first sold was from a common person whose produce is demai2Since the wholesaler sells in large quantities, he is permitted to sell demai (Chapter 11, Halachah 2). and the batch he sold later was from a chavair who made the appropriate separations. As we already explained,3Chapter 1, Halachah 6; Hilchot Terumah 5:12. we do not separate tithes from produce that is obligated to be tithed for produce that is exempt or from produce that is exempt for produce that is obligated. If the wholesaler said that [both batches] are from one person, we rely on his word.4Even though he is a common person. The rationale is that he has nothing to gain by lying.

הלכה ב
הָיָה מוֹכֵר כְּשׁוּת אוֹ יָרָק וְהֵן מְבִיאִין וְצוֹבְרִין לְפָנָיו הַלּוֹקֵחַ מִמֶּנּוּ מְעַשֵּׂר מִכָּל כְּשׁוּת וּכְשׁוּת וּמִכָּל אֲגֻדָּה וַאֲגֻדָּה וּמִכָּל תְּמָרָה וּתְמָרָה:
כסף משנה
2.
When [a wholesaler] sells hops or vegetables and they are being brought and collected before him, a purchaser should tithe each bunch of hops, each bundle [of vegetables], and each date individually.5The tithes cannot be separated from one for the other for the reason stated in the previous halachah.

הלכה ג
הַלּוֹקֵחַ מִבַּעַל הַבַּיִת וְחָזַר וְלָקַח מִמֶּנּוּ שְׁנִיָּה. מְעַשֵּׂר מִזֶּה עַל זֶה. אֲפִלּוּ מִשְּׁתֵּי קֻפּוֹת אֲפִלּוּ מִשְּׁתֵּי עֲיָרוֹת. שֶׁחֶזְקָתוֹ שֶׁאֵינוֹ מוֹכֵר אֶלָּא מִשֶּׁלּוֹ:
כסף משנה
3.
When a person purchases [produce] from a private individual and then purchases [the same species] from him a second time, he may separate the tithes from one batch for the other.6For each person may separate tithes from one batch of his own produce for another. Since the person who grows this produce has this option, it is also granted to one who purchases from him. [This applies] even when he purchases [produce] from two containers, and even from two cities. [The rationale is that] we operate under the presumption that a private person sells only his own produce.

הלכה ד
בַּעַל הַבַּיִת שֶׁהָיָה מוֹכֵר יָרָק בַּשּׁוּק בִּזְמַן שֶׁמְּבִיאִין לוֹ מִגִּנּוֹתָיו מְעַשֵּׂר מֵאֶחָד עַל הַכּל. וּבִזְמַן שֶׁמְּבִיאִין לוֹ מִגִּנּוֹת אֲחֵרוֹת אִם לָקַח מִמֶּנּוּ וְחָזַר וְלָקַח מִמֶּנּוּ פַּעַם שְׁנִיָּה לֹא יְעַשֵּׂר מִזֶּה עַל זֶה:
כסף משנה
4.
[Different rules apply when] a private person sells vegetables in the market place. When [produce] is brought to him from his gardens, [a purchaser] may tithe from one batch for the entire quantity. If, however, he is also brought [produce] from other gardens, should one purchase a batch from him and then purchase a second batch, he should not tithe from one for the other.7For it is possible that the second batch came from a different person and the difficulties mentioned in the first halachah might apply. This suspicion also applies in the halachot that follow.

הלכה ה
הַלּוֹקֵחַ פַּת מִן הַנַּחְתּוֹם לֹא יְעַשֵּׂר מִן הַחַמָּה עַל הַצּוֹנֶנֶת שֶׁאֲנִי אוֹמֵר חִטִּים שֶׁל אֶמֶשׁ הָיוּ מִשֶּׁל אֶחָד וְשֶׁל הַיּוֹם מִשֶּׁל אַחֵר:
כסף משנה
5.
When a person purchases bread from a baker, he should not tithe from the warm bread for the cool bread, for one might say that the wheat [used for] yesterday's [bread] was from one person and that [used] for today's is from another.8This applies even if the appearance of the bread is similar and they were baked in the same mold [the Rambam's Commentary to the Mishnah (Demai 5:3)].

הלכה ו
הַלּוֹקֵחַ מִן הַפַּלְטֵר אַף עַל פִּי שֶׁהֵן דְּפוּסִין הַרְבֵּה מְעַשֵּׂר מֵאֶחָד עַל הַכּל. שֶׁהַנַּחְתּוֹם שֶׁמּוֹכֵר לַמַּנְפּוֹל עוֹשֶׂה עִסָּתוֹ דְּפוּסִין הַרְבֵּה. אֲבָל הַלּוֹקֵחַ מִן הַמַּנְפּוֹל מְעַשֵּׂר מִכָּל דְּפוּס וּדְפוּס. שֶׁהַמַּנְפּוֹל לוֹקֵחַ מִשְּׁנֵי נַחְתּוֹמִין:
כסף משנה
6.
When a person purchases [bread] from a bakery9Our translation is based on the Rambam's Commentary to the Mishnah (ibid.). he may tithe from one loaf for all the others even if [the loaves] are from different molds. For a baker who sells to a bakery10Our version of the text is based on the Rambam's Commentary to the Mishnah (ibid.). The standard printed text of the Mishneh Torah follows a slightly different version. bakes his dough in different molds. If, however, one purchases [bread] from a distributor, one should tithe each mold individually,11When many loaves were made from the same mold, it is sufficient to separate tithes from one loaf for all those made in the same mold. One need not separate from each loaf individually, because we assume that all the loaves from the same mold came from the same baker. for a distributor purchases from two [or more] bakers.

הלכה ז
תִּשְׁעָה מַנְפּוֹלִין שֶׁלָּקְחוּ מֵעֲשָׂרָה נַחְתּוֹמִין הוֹאִיל וְאֶחָד מֵהֶן לָקַח מִשְּׁנַיִם כָּל הַלּוֹקֵחַ מֵאֶחָד מִן הַתִּשְׁעָה מְעַשֵּׂר מִכָּל דְּפוּס וּדְפוּס:
כסף משנה
7.
When nine distributors purchase [bread] from ten bakers, since one of them purchases from two [bakers], anyone who purchases [bread] from one of the nine must tithe each mold individually.12This applies even when the majority of the distributors bought their bread from one baker.

הלכה ח
הַלּוֹקֵחַ מִן הֶעָנִי וְכֵן הֶעָנִי שֶׁנָּתְנוּ לוֹ פְּרוּסוֹת פַּת אוֹ פִּלְחֵי דְּבֵלָה מְעַשֵּׂר מִכָּל אַחַת וְאַחַת. וּבִתְמָרִים וּבִגְרוֹגָרוֹת בּוֹלֵל הַכּל וּמְעַשֵּׂר. אֵימָתַי בִּזְמַן שֶׁהַמַּתָּנָה מְרֻבָּה. אֲבָל בְּמֻעֶטֶת מְעַשֵּׂר מִכָּל מַתָּנָה וּמַתָּנָה:
כסף משנה
8.
When a person purchases [bread] from a poor person and similarly, if a poor person was given13I.e., as charity. If he was given them as part of the presents given to the poor, he is not liable to make separations from them. slices of bread or slices of a cake of dried figs, he should tithe each one individually. For dates and dried figs, he should group the entire quantity together and tithe them.
When does the above apply? When he was given a large present.14Since the donor is generous, we assume that he also separated the tithes. Hence, the obligation to separate the tithes is merely a stringency. Therefore we are lenient with regard to its observance and do not suspect that the produce was given to the poor person by different people [the Rambam's Commentary to the Mishnah (Demai 5:5)]. If he was given a small present, he should tithe each present individually.15Since the donor was tightfisted, it is unlikely that he separated the tithes. Hence, the obligation to separate the tithes is more compelling and we are stringent with regard to the above suspicion (ibid.).

הלכה ט
שָׁחַק אֶת הַפַּת וַעֲשָׂאָהּ פֵּרוּרִין וְאֶת הַגְּרוֹגָרוֹת וַעֲשָׂאָן דְּבֵלָה מְעַשֵּׂר מֵאֶחָד עַל הַכּל:
כסף משנה
9.
If a person ground a loaf of bread [into] crumbs or ground dried figs and made them into a cake,16A loaf of bread and a cake of dried figs is considered as a large present. Hence the more lenient laws mentioned in the previous halachah apply (see Radbaz). he should separate tithes for the entire quantity.

הלכה י
פּוֹעֲלִים אוֹ אוֹרְחִים שֶׁהָיוּ מְסֻבִּין וְאוֹכְלִין וְהוֹתִירוּ פְּרוּסוֹת מְעַשֵּׂר מִכָּל אַחַת וְאַחַת: סְלִיקוּ לְהוּ הִלְכוֹת מַעשֵׂר בְּסִיַעְתָּא דִּשְׁמַיָּא
כסף משנה
10.
When workers or guests were reclining and eating and they left over slices [of bread], one should tithe each one individually.17We are speaking about the guests or workers of one person. Nevertheless, one cannot be lenient and separate tithes for the entire batch as one, for it is possible that one of the workers or guests separated the tithes and others did not and thus one might be separating tithes from produce from which tithes had been separated for produce from which they had not been. Although demai may be fed to guests (Chapter 10, Halachah 11), there may be some who desire to separate tithes as a stringency, but others will not.

זרעים הלכות מעשרות פרק יד
Zeraim Maaser Chapter 14