Terumos, Chapter 4, Mishnah 5. Our Mishnah comes to discuss the upper limit of the measure of Terumah: how much may a person give? We have already learned that Terumah has no lower limit - by Torah law a single kernel suffices to exempt the entire crop from the obligation of Terumah; and the Rabbis established measures: one sixtieth at the very least, an average person gives one fiftieth, and a generous person one fortieth of the crop.
The question before us is what the law is regarding someone who wishes to give more than that. Is there an upper limit?
Three opinions are stated on the matter, and the Mishnah opens: "HaMarbeh biTerumah" - that is, a person who wants to give beyond the upper measure of one fortieth that was established.
The opinion of Rabbi Eliezer - one tenth:
"Rabbi Eliezer omer: echad me'asarah" - up to a tenth of the crop can become Terumah Gedolah, but no more than that. The source of this measure: "kiTerumas ma'aser" - just as with Terumas Ma'aser one separates a tenth of the ma'aser, so too the maximum one can declare as Terumah Gedolah is one tenth of the crop. "Yoser mikan" - if he tries to give more than that, his declaration does not take effect on the additional amount.
For example: he had one hundred, and he wanted to separate thirty and declare the entire pile Terumah Gedolah. The most he can give is ten, which is a tenth of the hundred. It turns out that only ten out of the thirty take on the status of Terumah Gedolah, while the remaining twenty stay ordinary tevel from which nothing has been separated.
Here a practical problem arises: among the thirty, it is not known which are the Terumah and which are the tevel. Therefore he must find other produce from which ma'asros have not been separated, which can serve as ma'aser, and declare the remaining twenty to be Terumas Ma'aser for that quantity. In other words, since he has no way to determine which ten are the Terumah Gedolah and which twenty are not, he must turn the entire pile into Terumah and Terumas Ma'aser, by bringing an additional pile for which he will designate the Terumas Ma'aser contained in the thirty.
This is what the Mishnah says: "Ya'asenah Terumas Ma'aser lemakom acher" - he must designate the remaining twenty in this example as Terumas Ma'aser for produce from somewhere else.
The opinion of Rabbi Shimon - half and half:
"Rabbi Shimon omer: mechtzah chullin umechtzah Terumah" - in his view a person may give up to half of his entire crop as Terumah Gedolah. The source for this is the verse that obligates giving the first of the crop: "reishis deganecha". From here he learns that the first portion, which is the Terumah portion, cannot be greater than the grain portion, which is the portion that is not Terumah; and perforce the ratio is at most half and half.
The opinion of Rabbi Tarfon and Rabbi Akiva:
"Rabbi Tarfon veRabbi Akiva omrim" - as long as a person leaves over some portion that is not Terumah but chullin, his declaration of Terumah Gedolah takes effect in any amount whatsoever. Therefore a person may give ninety nine percent and more of his crop as Terumah Gedolah, provided he leaves some portion that is not Terumah Gedolah, and that is enough for the declaration to take effect. And this is the halachah, like Rabbi Tarfon and Rabbi Akiva.
In summary: In this Mishnah we learned the three opinions regarding the upper limit of Terumah: Rabbi Eliezer, who limits it to one tenth, like Terumas Ma'aser, and anything beyond that he should designate as Terumas Ma'aser for produce elsewhere; Rabbi Shimon, who permits up to half chullin and half Terumah, derived from the expression "reishis deganecha"; and Rabbi Tarfon and Rabbi Akiva, who hold that any amount takes effect, provided that he leaves over some chullin - and the halachah was decided in accordance with their view.