Commentaire sur Téroumot 5:3

תעלה ותאכל – In this, Rabbi Eliezer agrees that even that Seah is eaten, because of that above of ritually impure heave-offering that fell, even though it was suspended in pure heave-offering, nevertheless, when he causes heave-offering to be collected and it returns to its first designation, but here, when the pure [heave-offering] fell, even though the heave offering was ritually impure, when he collects it, it returned to the designation fo ritually pure heave-offering as it was.

Introduction Our mishnah deals with a case of clean terumah that falls into unclean hullin.

A seah of clean terumah fell into a hundred of unclean hullin, it may be eaten in small quantities, or roasted, or kneaded with fruit juice, or divided into pieces of dough so that the size of one egg be not in any one place. Since there is a 100-1 ratio in the mixture, the seah can be taken out, given to the priest and the remainder of the mixture reverts to being regular hullin. The seah given to the priest is not itself terumah, but rather it is given to him as a replacement for the terumah owed to him. Since he is eating it in place of terumah, he must try to preserve its purity and therefore he eats it in this way, as explained in mishnah one. In this case, Rabbi Eliezer does not disagree. When Rabbi Eliezer said that we look at the seah that is taken out as being the same seah that fell in, we say that in order to be stringent, as he was in the case of an unclean seah of terumah that fell into clean hullin. In our case, were we to say that the seah that fell in was the same that was taken out, then he could eat it without trying to maintain its purity. Since this would be a leniency, Rabbi Eliezer agrees that we don’t say this.