Terumot, Chapter One, Mishnah Five
Most of this mishnah teaches that one cannot take terumah from one category of produce in order to exempt another category of produce.
1) They do not take terumah from gleanings, from the forgotten sheaf, from peah or from ownerless produce.
2) [Neither is it taken] from first tithe from which terumah had already been taken, nor from second tithe and dedicated produce that had been redeemed.
3) [Nor is it taken] from that which is subject [to terumah] for that which is exempt [from terumah], nor from that which is exempt for that which is subject.
4) Nor from produce already plucked [from the soil] for that attached to it, nor from that attached [to the soil] for that already plucked.
5) Nor from new produce for old, nor from old for new.
6) Nor from produce from the land of Israel for produce grown outside the land, nor from that grown out of the land for that grown in the land.
7) [In all these cases] if they did take terumah, their terumah is not terumah.
Section one: All of the agricultural gifts given to the poor are exempt from terumah and tithes. When a poor person receives these gifts he need not take out terumah or tithes. For an explanation of what these gifts are see the introduction to peah. Similarly, ownerless produce is exempt from terumah. Furthermore, as we shall learn below, one cannot separate terumah from that which is exempt on behalf of that which is liable for terumah.
Section two: When the Levite receives tithe (first tithe), he takes out terumat maaser and gives it to the priest (see the introduction). Even if the tithes that he received came from produce that never had regular terumah taken out of it, for instance the Levite took his tithes earlier than was normal (usually terumah is taken first and then the tithes), it is not subject to regular terumah. The same is true for second tithe and sanctified produce that has been redeemed (exchanged for moneyin the case of second tithe the money is then taken to Jerusalem, and in the case of sanctified produce the money is given to the Temple). Even if terumah has never been taken from such produce, it is no longer subject to the laws of terumah.
Section three: If one has two piles of produce, one pile from which he must remove terumah, and one pile from which he does not have to remove terumah, he cant take terumah from one in order to exempt the other. Specifically, he cant take from the exempt produce in order to have it count for the liable produce.
Section four: Produce that is still attached to the ground is not yet liable to have terumah taken from it. Therefore, one cannot remove terumah from produce already plucked in order to exempt produce that is still attached, nor vice versa.
Section five: New produce (from this years harvest) and old produce (meaning aged produce from the previous years harvest) are different categories. One cannot take terumah from one in order to exempt the other.
Section six: Produce grown outside of the land of Israel is not subject to the laws of terumah. Therefore, one cannot take terumah from such produce in order to exempt produce that grew inside the land, nor vice versa.
Section seven: In all of these cases, if one does separate such terumah it is not considered terumah and even a non-priest could eat it.