Terumot, Chapter Four, Mishnah Three
As I stated in the introduction the Torah does not prescribe how much terumah a person must give as it does for tithe. Our mishnah gives recommendations as to how much a person should give.
1) The amount of terumah:
a) A generous amount: one fortieth. Beth Shammai say: one thirtieth.
b) The average amount: one fiftieth.
c) A stingy amount: one sixtieth.
2) If he gave terumah and discovered that it was only one sixtieth, his terumah is valid and he need not give again.
3) If he does go back and add to it, [the extra amount] is liable to tithes.
4) If he found that it was only one sixty-first it is valid, but he must give terumah again according to his established practice, in measure, weight or number.
5) Rabbi Judah says: even if it be not from produce close by.
Section one: This section lists various percentages of terumah that a person might give, depending on his generosity. In the mishnah Bet Shammai disagrees only concerning the definition of a generous amount. In the Tosefta, Bet Shammai adds that an average amount is one fortieth, and that a stingy amount is one fiftieth.
Section two: If he intended to give an average or above average amount and he gave only one sixtieth, considered a stingy amount, it still counts and he has fulfilled his duty. He need not give any more terumah.
Section three: If despite this he does try to give more terumah, the extra terumah that he gives is not considered terumah. It is still untithed produce, and before it can be eaten, tithes must be removed. This section teaches us that if one gives one sixtieth, he has fulfilled his obligation and he cant add to it later on.
Section four: This is the opposite scenario. He thought that he had given a sufficient amount, but it turns out he gave less than one sixtieth. He now must give terumah again and when he gives he should give according to his established practice. If he usually gives 1/40, he should give that amount, etc. In addition, there are a couple of leniencies that apply to this situation. First of all, he can give by measuring the amount he is giving, although usually one is not supposed to give terumah in this manner (see 1:7).
Rabbi Judah allows him to give from produce that is not nearby, meaning that it is not close to the original pile of not fully tithed produce. Normally one should give terumah from produce that is close to the produce for which he is separating.