Maaser Sheni, Chapter 3, Mishnah 3. This mishnah deals with a person who has coins of Maaser Sheni in Jerusalem and needs money for other purposes: how can he transfer the sanctity of the maaser onto his friend's produce and use the coins for his own needs.
The text of the mishnah and its explanation:
"Mi shehayah lo maos b'Yerushalayim v'hu tzarich lahem" - a person who has coins of Maaser Sheni in Jerusalem, and he needs them for purposes other than eating. After all, for eating one may use Maaser Sheni money at any time, whereas here he needs the coins to buy furniture or for other needs for which Maaser Sheni money may not be used.
"Ul'chavero peiros" - and his friend has produce of chullin, also in Jerusalem.
"Omer l'chavero: harei hamaos ha'elu mechulalin al peirosecha" - he redeems the maaser coins in his possession onto his friend's produce, and thereby the produce takes on the sanctity of Maaser Sheni.
"V'nimtza zeh ochel peirosav b'taharah" - his friend, who agrees to this, will now be required to eat his produce in a state of taharah, because it has taken on the status of Maaser Sheni, and Maaser Sheni must be eaten in a state of taharah.
"V'halah oseh tzorko b'maosav" - and the owner of the coins may use them for his own needs, such as buying furniture, since they are no longer Maaser Sheni money.
The limitation - "V'lo yomar kein l'am ha'aretz ela b'demai":
If the friend who holds the produce is an am ha'aretz, this procedure may be carried out only if the coins in his possession are Maaser Sheni coins of demai - that is, Maaser Sheni that was separated from produce purchased from an am ha'aretz, where it is uncertain whether maaseros were separated from it, and therefore Maaser Sheni must be separated from it again. When such produce is redeemed onto coins, the coins take on the status of 'Maaser Sheni coins of demai.' Only with these may one carry out the transaction with an am ha'aretz when he needs his money.
The reason: since there is no certainty that the am ha'aretz will eat the produce in a state of taharah, we are concerned that he may not eat it that way, and if he were given money of definite Maaser Sheni, that would be a problem. But with Maaser Sheni of demai, which is not definite Maaser Sheni, there is room to be lenient.
Why were they lenient with demai?
Most amei ha'aretz do separate maaser: we have an established principle that most amei ha'aretz do in fact separate maaseros. Therefore it can be assumed that this separation was not needed at all, so Maaser Sheni coins of demai are not really Maaser Sheni coins to begin with, and they carry no sanctity from the outset.
A double doubt (sfek sfeika): some explain that the leniency is based on a double doubt. The first doubt: perhaps the am ha'aretz from whom the produce was bought did in fact separate maaseros, in which case there was no need to separate Maaser Sheni at all, and the produce has no status of Maaser Sheni. The second doubt: perhaps the am ha'aretz himself will eat it in a state of taharah, even if it is definite Maaser Sheni. Because of this double doubt, one may be lenient with coins of Maaser Sheni of demai.
In summary: in this mishnah we learned that someone who has Maaser Sheni coins in Jerusalem and needs them for his own purposes may transfer their sanctity onto his friend's produce of chullin: the produce takes on the sanctity of Maaser Sheni and must be eaten in taharah, and the coins revert to being chullin. However, one may do this with an am ha'aretz only with coins of Maaser Sheni of demai, because of the concern that he will not eat the produce in taharah - and with demai they were lenient, both because most amei ha'aretz separate maaser and there is no sanctity here from the outset, and because of the double doubt.