Comentario sobre Terumot 4:3

עין יפה אחד מארבעים – He who is prompted by the heart and donates the heave-offering generously, separates one in forty.

The amount of terumah: A generous amount: one fortieth. Beth Shammai say: one thirtieth. The average amount: one fiftieth. A stingy amount: one sixtieth.
If he gave terumah and discovered that it was only one sixtieth, his terumah is valid and he need not give again.
If he does go back and add to it, [the extra amount] is liable to tithes.
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.
Rabbi Judah says: even if it be not from produce close by.
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.
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 can’t 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.

והבינונית אחד בחמשים – these are not the words of the School of Shammai, but rather, the first Tanna, who stated it. But from the generosity of the School of Shammai (who said one in thirty) you learn for their words what is selfishness/the evil eye and the middle/intermediate amount, even though they are not taught [explicitly] in our Mishnah.

ועלה בידו אחד מששים – as, for example, that he donated from an estimate/guess-work, and there came up into his hand one out of sixty which is selfishness/the evil eye.

חזר והוסיף חייב במעשרות – for he has already exempted and the designation of heave-offering does not fall upon what he adds, therefore, this addition is completely eatables forbidden pending the separation of sacred gifts , and he is liable for tithes.

מששים ואחד – as, for example, that there were sixty one Seah in the pile and he took up a Seah in his hand, that Seah is the heave-offering/Terumah.

ויחזור ויתרום כמו שהוא למוד – [if he was accustomed] to donate one from sixty, he should still separate one from sixty in a Seah, and if he was accustomed to donate one from fifty [Seah], and there were found that there still remained eleven Seah that had not been donated [for Terumah].

במדה ובמשקל – meaning to say, that excess/surplus that he donates [as Terumah], it is permissible to donate/separate out Terumah by measure, or by weight or by number. For they didn’t say to donate/separate Terumah] from an estimate, other than at the beginning of the heave-offerinf.

אף שלא מן המוקף – for Rabbi Yehuda holds that one does not need to donate [Terumah] from that which is near and adjoining, other than at the beginning of Terumah. But the Halakha is not according to Rabbi Yehuda.

מוקף – near and adjoing, and in the same place iself.