Terumot, Chapter Four, Mishnah Five
In mishnah three we learned the minimum amounts that one can give as terumah. Today we learn the maximum amounts.
One who wishes to give more terumah:
1) Rabbi Eliezer says: he may give up to a tenth part, as in the case of heave-offering of tithe. [If he gave] more than this [measure] he must make it terumah of tithe for other produce.
2) Rabbi Ishmael says: half non-sacred produce and half terumah.
3) Rabbi Tarfon and Rabbi Akiba say: as long as he retains a part as non-sacred produce.
Section one: Rabbi Eliezer says that one cannot give more than one tenth of ones produce as terumah. This is equivalent to the amount of terumah that the Levite separates and gives to the priest from the tithe that he receives.
If he gave more than this, then the extra amount doesnt count as terumah. Rather it is considered untithed and one must tithe it before it is eaten. However, he cant give any of it to the Levite because there is terumah mixed in with it (the ten percent does count as terumah). So what he can do is let the Levite use the entire amount that he separated as terumat maaser (terumah taken from first tithe) for other produce that he has received as tithe. In this way it will all become terumah.
Section two: According to Rabbi Ishmael, he can give half of the pile as terumah.
Section three: Rabbi Tarfon and Rabbi Akiva allow him to give as much as he wants, as long as some of the pile remains non-sacred produce.