Terumos, Chapter 6, Mishnah 5. We learned that one who eats Terumah unintentionally pays for what he ate plus an added fifth, but in practice his obligation is to restore that very Terumah itself. We can picture this as a kind of preservation of the Terumah - it cannot be lost. Once the Terumah has been consumed, the one who ate it must bring new tithed chullin, ordinary food that has been properly tithed, and give it to the kohen; and from the moment it is given to the kohen, it becomes Terumah by Torah law, and so too the payment of the fifth becomes Terumah by Torah law. The source of this is the verse: "v'nasan la'kohen es hakodesh" - the act of giving to the kohen restores the item and sanctifies it once again as Terumah.
The derivation from the verse rests on the words "es hakodesh": one may only give the kohen sanctified food, food that is fit to become sanctified as Terumah - that is, food that in its history was of the type that could have become Terumah, except that in practice it did not, and it remained tithed chullin. Our Mishnah comes to list a number of halachic statuses of foods on account of which the food is disqualified from serving as the payment that replaces the Terumah and adds a fifth to it.
Four foods that were never obligated in tithes:
The Mishnah opens with a list of four categories of food that were never obligated in tithes at all, and therefore could never have become Terumah Gedolah. The first three are the gifts to the poor that are left in the field during the harvest, and they are exempt from tithes: the poor person who is entitled to take them need not tithe them, and he eats them as they are. Since they were never fit to become Terumah, they are not fit to be given as payment by someone who ate Terumah unintentionally.
In the words of the Mishnah: "ein meshalmin min haleket umin hashich'chah umin hape'ah" - one cannot discharge the debt and restore the Terumah, the principal and the fifth, from these gifts. They are not fit to become sanctified, since they do not meet the requirement derived from the verse - that they be of the type of food that can become Terumah.
Leket - the stalks that fall during the harvesting of the grain. If one or two stalks fall, they must be left for the poor to gather.
Shich'chah - the sheaves that were forgotten or overlooked. When a person ties the stalks of grain into sheaves and brings them to the threshing floor, if he forgot one or two sheaves, he must leave them, and he may not go back and take them. They remain as shich'chah, and the poor come and collect them.
Pe'ah - the edge of the field that the owner must leave unharvested, and under ordinary circumstances its measure is one sixtieth of the field, so that the poor will come and harvest the corner of the field left for them. This food too is exempt from tithes.
To picture the situation: a poor person ate Terumah by mistake, and he must now pay the principal and the fifth. What does he have in his pantry? The gifts he gathered in the field. These foods are exempt from separating Terumos and maasros, and therefore he cannot give them to the kohen as the compensation payment, since they could never have become Terumah. Instead he must go out to the market and buy - if he ate grapes, he buys other grapes from which Terumos and maasros have been separated - and gives these tithed grapes to the kohen as payment of the principal and the fifth. The gifts he gathered himself he may not use.
The fourth item on the list, "umin hahefker", is likewise forever exempt from the obligations of tithes. Hefker is ownerless property. And as we have seen, in order to become obligated in tithes the food must be guarded and cultivated by a person and designated for human consumption; but if it grew on its own in nature and has no owner, it is forever exempt. There are two cases here:
The primary case: a person walks into an open field and finds a wild fig tree - he eats its figs and never needs to tithe them.
The secondary case: a person cultivated a fig tree, but declared the figs ownerless before the obligation of tithing took effect on them. Exactly when that obligation takes effect we will learn in the next tractate, Maasros; in any case, if he declared them ownerless before the obligation took effect, they are exempt from tithes forever.
Foods sanctified with a different sanctity - the dispute between Rabbi Meir and the Sages:
Now the Mishnah lists three additional categories, which went in the opposite direction in the separation process: foods that were separated as tithes of other kinds, or that became sanctified with some sanctity in another way. Regarding these, Rabbi Meir and the Sages disagree. It is worth noting that this Mishnah is extremely challenging, since it is not at all clear exactly which items are listed by Rabbi Meir as disqualified, and which of them the Sages permit. We will learn it as usual according to the Bartenura, though it is not simple.
First tithe from which its Terumah was taken: "v'lo mima'aser rishon shenitlah terumaso" - one may not use first tithe from which its Terumah has been separated as compensation payment for Terumah that was eaten unintentionally. The case is a person who, after separating Terumah, separated his first tithe - the ten percent that is given to the Levi. This tithe may never be used as compensation for eating Terumah unintentionally.
Suppose a Levi ate Terumah by accident. The Levi receives his ten percent, separates a tenth of it - which is one percent of the original quantity - and gives it to a kohen as terumas maaser. The remaining nine tenths stay with him to be eaten as ordinary food in every respect. Even so, the Mishnah rules that those nine tenths, the Levi's maaser from which its terumah has already been separated, are invalid for use as the principal and fifth compensation payment.
The Bartenura explains that once it became invalid, it is invalid forever. Since it was designated as maaser, and while it was maaser the terumah was still absorbed within it so that it was not fully tithed, it can never again become fit, even after all the required tithes have been separated from it and its terumas maaser has been taken.
The question embedded here, which all the commentators wrestle with, is perhaps better than the answer given to it: it is hard to pin down the precise difference between ordinary tevel - produce that was harvested and became obligated in tithes, and once the tithes were separated from it became complete chullin and valid for payment - and maaser rishon, which likewise becomes fully tithed once its terumas maaser is taken. Both may now be eaten by anyone. Many approaches have been offered here, depending on how one understands the restriction inherent in maaser, its status, and whether a sanctity is invested in it.
Let us quote the language of the Bartenura Mizrachi, who acknowledges the difficulty that properly tithed chullin and fully tithed maaser look like one and the same thing: "Ve'af al gav de'hashta chashivin chullin metukanim" - even though the fully tithed maaser is now exactly like properly tithed chullin, "keivan de'kodem she'nitlah terumaso lo chazi" - since before the terumas maaser was taken it was not fit to serve as a compensation payment, "hashta nami lo chazi" - now too it is not fit, and it remains permanently invalid for payment for eating Terumah by accident. We will leave this as a question requiring further study, and you can look further into the many approaches that resolve the difference between fully tithed maaser and properly tithed chullin.
Maaser sheni that was redeemed: Maaser sheni is the tithe taken after terumah gedolah and maaser rishon have been separated, in years 1, 2, 4 and 5 of the shemittah cycle - as opposed to years 3 and 6, in which maaser ani is taken. Maaser sheni is eaten by the owners of the produce themselves, except that eating it is restricted to Jerusalem. According to Rabbi Meir, who is the speaker here, maaser sheni is the property of the Most High - as it were, the food of Hashem, and He invites a person to sit at His table and eat of His food in Jerusalem. In any case, a certain sanctity is rooted in it, and therefore it is disqualified from serving as payment for Terumah.
The case is maaser sheni that was redeemed: before it was brought to Jerusalem it could be redeemed, removing its sanctity from the food and transferring it onto coins, and with those coins one goes up to Jerusalem and buys food there. In this way the food that had been maaser sheni is left completely deconsecrated and is truly chullin. Even so, just like maaser rishon that was fully tithed - since at some stage this sanctity applied to it and it was not valid for paying for Terumah, it remains that way forever.
Hekdesh that was redeemed: Hekdesh is anything given to the Beis HaMikdash and consecrated for its use, and the case here is hekdesh for the upkeep of the Temple building. A person who had figs and decided to donate them to the Beis HaMikdash - the Beis HaMikdash receives the figs, and if it has no use for them, they are converted into money by selling them and the money is used for the maintenance of the building, or the workers are fed with them directly, all as they see fit. The point is that the figs are saturated with sanctity, and a person may not misuse hekdesh in any way from the moment he consecrated them.
These figs can be redeemed: when the Beis HaMikdash converts the figs into money by selling them, the money with which the buyer purchases them deconsecrates the figs and renders them fully edible as ordinary figs that have gone out to chullin, while the sanctity is invested in the money the Beis HaMikdash receives. This is how sanctity can be redeemed from hekdesh, in the case of hekdesh for the upkeep of the Temple building, and that is our case here. Therefore, a person who bought figs from the Beis HaMikdash, deconsecrated them and made them fit to eat, and afterward ate Terumah figs by accident and is obligated to pay the principal and a fifth - he cannot pay with the figs he bought from hekdesh. The reason is that sanctity was invested in them, and according to Rabbi Meir, once that sanctity disqualified them, they remain permanently disqualified.
In the words of the Mishnah: "Ve'lo maaser sheni ve'hekdesh she'nifdu" - one may not pay from maaser sheni or from hekdesh that were redeemed. And the reason for the disqualification is stated explicitly in the Mishnah: "She'ein hekdesh podeh es hekdesh" - something that is, or once was, consecrated and holy cannot serve to redeem hekdesh.
This is Rabbi Meir's position, and the Sages disagree with him: "Va'chachamim matirin be'eilu". However, the commentators differ as to which items on the list the Sages are referring to:
According to the Bartenura: the Sages are referring only to maaser sheni and hekdesh that were redeemed - the last two items on the list, those that were explicitly consecrated as maaser sheni and as hekdesh and were afterward redeemed.
Some expand this: the Sages permit even maaser rishon whose terumah was taken.
And some expand it further: the Sages permit even the gifts to the poor.
The halachah certainly follows the Sages, but the question remains as to which of the items on the list they permitted. For now we are left with the straightforward reading of the Bartenura.
In summary: Our Mishnah teaches from which foods one does not pay the principal and the added fifth for eating Terumah unintentionally, based on the derivation from "venasan lakohen es hakodesh" - a food that is fit to become sanctified as Terumah. First it listed four items that were never subject to tithes: leket (gleanings), shich'chah (forgotten sheaves), pe'ah (the corner of the field) and hefker (ownerless produce). After them it listed three that had already been sanctified with a different sanctity: first tithe from which its Terumah was taken, second tithe that was redeemed, and consecrated property that was redeemed - which according to Rabbi Meir remain disqualified forever, while the Sages permit them, and the commentators disagree as to the scope of that permission.