Terumos Chapter 3, Mishnah 6. We now move on to a new topic: the correct order of the various gifts that a person separates from his agricultural produce. Our Mishnah focuses on four stages, three of which are already familiar to us: Terumah Gedolah, Maaser Rishon and Maaser Sheni - and in the years when there is no Maaser Sheni, Maaser Ani.
Why the Mishnah does not deal with Terumas Maaser:
Terumas Maaser is a gift that is not necessarily given by the farmer who worked the field, but rather by the Levi who receives the Maaser from the farmer. There is therefore no need to fix a precise order for separating it: the Levi is responsible for separating Terumas Maaser when he receives the Maaser, while the farmer is responsible for separating Maaser Sheni - but only after Maaser Rishon. It makes no difference, then, who comes first: the Levi giving Terumas Maaser or the farmer giving Maaser Sheni. What is essential is that Terumah Gedolah, Maaser Rishon and Maaser Sheni (or Maaser Ani, as the case may be) be taken in that order.
The fourth element - Bikkurim:
The Mishnah adds a fourth element: Bikkurim. This is a topic in its own right, and there is an entire tractate called Bikkurim which we will reach, God willing, before long. The basic idea is that with the seven species for which the Land of Israel is praised - wheat and barley, olives, dates, grapes, figs and pomegranates - a person is required to take the first fruits and grain, those that ripen earliest, bring them to the Beis HaMikdash and give them to a kohen, where a special procedure is performed with them.
How is this done in practice? Suppose a person is in the wine business and owns vineyards. He goes out early in the season, notices the first clusters of grapes beginning to grow, and marks them as ones that will be given in the Beis HaMikdash as Bikkurim. He ties a red string around those tiny, developing clusters, so that he will be able to identify them weeks or months later at harvest time, and those grapes will be the Bikkurim. It turns out that Bikkurim comes before Terumah Gedolah, which happens much later - after the grapes are pressed and brought as wine, in order to give the portions to the kohen, the Levi and so on. This, then, is the very first stage of all.
The order of separation:
Bikkurim.
Terumah Gedolah.
Maaser Rishon.
Maaser Sheni or Maaser Ani.
The Mishnah rules that these four things must be done in their proper order. One who does not do so has acted improperly, since the Torah asks that it be done in order, and he transgresses a negative commandment. Nevertheless, the separation itself, the declaration that this item shall be what it is designated to be, takes effect after the fact even though it was done out of order.
The words of the Mishnah:
"HaMakdim Terumah LeBikkurim" - one who gives the Terumah Gedolah before the Bikkurim.
"Maaser Rishon LiTerumah" - one who separates Maaser Rishon for the Levi before the Terumah for the kohen.
"Maaser Sheni LaRishon" - one who gives Maaser Sheni, meaning the items he will take up with him to Jerusalem, before he has separated Maaser Rishon for the Levi.
About all of these the Mishnah says: "Af al pi shehu over belo saaseh" - even though he has transgressed a Torah prohibition - "mah she'asah asuy". He did the wrong thing and violated the prohibition, but what is done is done: if he separated Maaser Rishon before Terumah Gedolah, that Maaser Rishon indeed becomes Maaser Rishon for the Levi, and the separation takes effect.
The source of the law: "shene'emar: mele'ascha v'dim'acha lo se'acher" - the fullness of your harvest and your Terumah you shall not delay. This is a full-fledged derashah, in which "mele'ascha" refers to Bikkurim and "dim'acha" to Terumah (and we have already encountered a related word - 'meduma'). We will leave aside here the technical details of how the derashah works, but it is clear that the verse refers to Bikkurim and Terumah and teaches that these must be done in this order.
It is worth noting that even though the separation takes effect, doing it out of order carries other consequences, among them that he will not be able to recite viduy ma'asros at the end of the processing season.
This is the Mishnah, and this is the halachah.