Here is the second mishnah in chapter 5, mishnah 4, which deals with the laws of "lifnei iver" (placing a stumbling block before the blind) in the context of separating tithes: a person who sells his production leftovers to someone who is not trusted regarding tithes.
The case in the mishnah:
A person sells the waste left over from his production. The common assumption is that the buyer is not purchasing it in order to extract further edible produce from it, but for other purposes: fertilizing the field, animal feed, or fuel. For that reason, under ordinary circumstances the seller need not be concerned that the buyer will extract a few kernels from the straw and eat them. Were there such a concern, and the buyer could not be relied upon to separate tithes properly, the seller would transgress "lifnei iver," since he would be placing a stumbling block before someone known not to keep the laws of tithes.
However, if the buyer explicitly announced that he intends to extract an edible portion from the waste, the laws of "lifnei iver" come back into force. The reason for this is stated later in the mishnah: one who extracts edible portions from the leftovers - such as kernels remaining in the threshing floor waste - is obligated to separate tithes from them.
The mishnah rules: "lo yimkor adam es tivno" - a person may not sell his straw, and likewise his olive pulp and his grape husks. These are the three types of waste that the mishnah addresses:
Teven - this is straw, and it should not be confused with hay. Hay is the whole stalk with the ear at its top, which contains the edible part, and it is intended for animal feed (alfalfa, various grasses, barley and the like). Straw itself has almost no nutritional value, since the nourishing parts - the seeds - have already been separated from it, and today its main use is as bedding for animals.
Gefes - the waste remaining after olives have been pressed, familiar also from the mishnah in the third chapter of Maseches Shabbos that deals with heating ovens. Generally the olive pulp serves as a fuel source and not for producing oil.
Zagin - the grape skins remaining after the grapes have been trodden in the winepress. They too are not fit for eating, and generally serve for fertilizer or animal feed.
Why, then, is there any concern that food might be extracted from the straw? Because the separation process is never perfect. A person harvests the wheat stalks and brings them up to the threshing floor, where he threshes them and separates the kernels from the chaff, and afterward winnows them with a pitchfork: the light straw is carried off by the wind to one side, while the kernels gather on the other side, cleaned and stored for eating. Even so, a few kernels always remain within the straw - ones that were not properly threshed, or that got mixed into the chaff and were not extracted in the winnowing.
The rule that emerges: if the buyer said explicitly that he intends to extract edible food from the straw, to draw the last drops of oil from the olive pulp for eating, or to make grape juice from the husks - and you do not trust him to separate tithes properly - you must be concerned about "lifnei iver," and you may not sell to him. But if he did not reveal to you what he intends to do with them, there is no need for concern, and it is permitted to sell to him even though he is not trusted regarding tithes.
"im hotzi mehen - chayav b'maasros u'perurin min haterumah": - if he extracted from them, he is obligated in tithes and exempt from Terumah.
The mishnah goes on to state explicitly: one who extracts edible portions from the waste - wheat kernels, wine, or olive oil - is obligated to separate tithes from them, but is exempt from separating Terumah gedolah from them.
The reason for the exemption from Terumah gedolah rests on two principles. First, in the laws of Terumah gedolah there is no fixed measure, and even the smallest amount exempts the entire crop. Second, Terumah is not measured out precisely but is separated by estimate - about two percent, one in fifty for an average person, and not with exactitude. For this reason the Sages said that one separating Terumah should have in mind the entire crop: all the wheat, including even the wheat that never reached the main pile brought into the storehouse, but rather remained off to the side or got mixed into the chaff and never properly came out of the threshing floor.
Moreover, the Sages not only established that this is the proper thing to do, but that this is in fact what actually happens: an ordinary person separating Terumah, who did not have anything specific in mind, has all of these in mind. Only if he explicitly intended to exclude them from his Terumah, and said that he is deliberately leaving the threshing floor's grain without designating any portion of it for Terumah gedolah, are they not included. And since ordinarily he has them in mind, there is no obligation to separate Terumah gedolah from leftovers that remained in the field or within the straw and stubble after Terumah gedolah has already been separated.
All of this applies only to Terumah Gedolah. It does not apply to maaser rishon - the gift given to the Levi'im - nor to maaser sheni and the terumas maaser that follows it. The obligation of maaser rishon is a precise measure of one tenth, and it cannot be estimated by approximation or made to include what is not in front of you: one must separate a tenth of what is actually in his hand, and whatever was left behind remains without maaser having been separated from it.
A closer look at the wording of the Mishnah:
The Mishnah gives the reason for the exemption from Terumah: "shehatorem b'libo al haketumin v'al hatzedadin umah shb'toch hateven" - because one who separates Terumah has in mind the broken kernels, the ones at the sides, and whatever is inside the straw. The three expressions mean:
"haketumin" - literally, the broken ones, referring to the portion of grain that was not properly threshed.
"hatzedadin" - the grain that lay at the sides of the pile and never made it into the main heap when the grain was gathered.
"mah shb'toch hateven" - the kernels that remained mixed in among the straw.
With this last expression the Mishnah comes full circle. It opened by discussing the sale of straw, which in itself is of no use as human food, even though a few kernels remain within it. Since the one separating Terumah intends his separation to cover every kernel still mixed into the straw, when he sells that straw to another person and the buyer wishes to extract the kernels from it, there is no reason to be concerned that Terumah was never taken from them - for Terumah was certainly separated on their behalf together with the rest of the crop.
In summary: In this Mishnah we learned that a person need not be concerned about 'lifnei iver' when selling straw, olive pulp or grape residue to someone who is not trustworthy regarding tithes, since the ordinary buyer does not take them in order to produce food; but if the buyer stated explicitly that he intends to extract food from them, the sale is forbidden. We also learned that one who does extract food from them is obligated in tithes, because their measure is fixed at one tenth, and is exempt from Terumah Gedolah, because the one separating Terumah had in mind from the outset the broken kernels, the ones at the sides, and whatever is inside the straw.