Terumos chapter 11, mishnah 6. This mishnah too deals with the requirements surrounding the loss of Terumah and with the general approach we have been developing: a person must handle the Terumah in his possession in the ordinary, accepted manner. The case before us is a storehouse that was used for storing Terumah, which has been emptied out, and its owner now wishes to store non-sacred produce in it. The question is: how far must a person go to make sure that not even a single grain of Terumah remains on the storehouse floor?
The rule in the laws of mixtures:
Generally speaking, when it comes to mixtures a great deal of care is required, since the rule is that one may not intentionally nullify a prohibited substance. A person is not at liberty to treat the matter lightly and say: what difference does it make? If he scatters crumbs of a forbidden food into his dish in order to add flavor to it, claiming that the dish contains sixty times as much as the crumbs, this is not permitted at all. It is a prohibition, and apparently a Torah prohibition, at least in a mixture of dry items mixed together, and possibly even in a liquid mixture. Ideally one may not do this.
And what about Terumah?
The language of the mishnah indicates that here they did not impose any extra stringency on a person:
"Megurah shepinah mimenah chitei terumah" - a storehouse in which wheat of Terumah had been stored, and the owner cleared it out.
"Ein mechayvin oso leishev u'lelaket achas achas" - this is a rabbinic dispensation in which the Sages were lenient: we do not require the owner to sit down on the floor and gather up every last grain of Terumah.
"Ela mechabed kedarko" - it is enough that he sweep the place the way one normally sweeps, a thorough sweeping, and that suffices.
"V'nosen l'sochah chullin" - and he may bring the non-sacred grain in there, even though he knows full well that a few grains of Terumah wheat remain in the place and will become mixed in with it.
The reason for the leniency: the alternative would have placed an impossibly demanding requirement on the farmer, and the Sages did not demand that of him.
In summary: even though the laws of mixtures call for great care, and one may not intentionally nullify a prohibited substance, with Terumah the Sages were lenient: one who clears a storehouse of Terumah wheat is not required to gather grain after grain, but rather sweeps the place in the usual manner and brings non-sacred produce in - for we do not obligate a person with a demand that he has no ability to meet.