Demai, Chapter Five, Mishnah Eleven
This mishnah deals with giving terumah from demai produce for produce that has certainly not been tithed, or vice versa, from certainly non-tithed produce for demai. The problem here is similar to the problem in the previous mishnah. He is giving terumah for produce that is only liable derabbanan (demai) on behalf of produce that is liable from the Torah, the non-tithed produce.
1) If one gave terumah from [produce of] demai for [other produce of] demai, or from [produce of] demai for [produce] which was certainly untithed, [this becomes] terumah, but he must give terumah over again.
2) [If he gave terumah] from [produce] which was certainly untithed for [produce of] demai, [this becomes] terumah, but it may not be eaten until he first gives terumah and tithes for it.
Section one: In this case he separates terumah from produce that is only doubtfully liable for tithes on behalf of other produce that is either liable for tithes (non-tithed produce) or only liable derabbanan (the other demai). As we have seen, that which he calls terumah is terumah, but he still must again go back and separate terumah from the other demai or non-tithed produce. In short, the separation of terumah was not effective to render the produce permitted.
Note that in the case of separating from demai on behalf of other demai the mishnah does not state that before this terumah can be eaten he must go back and separate again from the first demai, lest it had not been tithed and the second demai was tithed and it turn out that he was separating from obligated produce on behalf of non-obligated produce. According to the Yerushalmi, the rabbis were not strict in this scenario.
Section two: In this case he gave terumah from produce that is certainly liable on behalf of other produce that is doubtfully liable (demai). As we saw at the end of yesterdays mishnah, in such a scenario he has not successfully separated terumah from the first produce (here the non-tithed produce), and also not from the produce for which he was separating (here the demai). That which he calls terumah is still terumah, but in order for a priest to eat it he will have to go back and separate terumah and tithes from the non-tithed produceuntil then the produce is still considered non-tithed, and forbidden to all.