Ma'aser Sheni, Chapter 4, Mishnah 6. The Mishnah deals with someone who is in the middle of the process of redeeming Ma'aser Sheni by exchanging it with another person, and in the middle of the process the value of the produce changes. How is the monetary obligation determined, and how is the amount of the redemption determined?
Two separate stages in the process:
This is not a sale but a redemption: a person gives ordinary (chullin) money in exchange for the Ma'aser Sheni produce, the money becomes Ma'aser Sheni money, and the produce becomes chullin in the hands of the one who received it. In this process there are two points in time that must not be confused:
The acquisition of the produce - takes place through meshichah (pulling), that is, at the moment he actually takes the produce into his possession. The monetary obligation is determined according to this moment.
The redemption - takes place only at the moment the money is handed over. The amount of the redemption is determined according to this moment.
The first case - the value went up:
"Mashach mimenu ma'aser b'sela v'lo hispik lifdoto ad she'amad bishtayim" - he performed an act of acquisition on the produce when it was worth a sela, but he did not manage to hand over the money before its value doubled and stood at two sela. It turns out that as far as the monetary obligation is concerned he owes only a sela, which was the value at the time of the meshichah; but as far as the redemption is concerned, the Ma'aser Sheni is redeemed according to its value at the time of the redemption - two sela.
Therefore "nosein lo sela u'mistakeir b'sela u'ma'aser sheni shelo" - he gives his fellow the sela that he owes him, and that sela becomes Ma'aser Sheni money. The second half of the value he must redeem with an additional sela, except that this sela remains in his own possession: he designates a coin of his own as Ma'aser Sheni money and uses it himself, and he is not required to give it to his fellow. It turns out that he gained produce worth a sela - he holds produce worth two sela, and he also holds a Ma'aser Sheni coin that remains his.
The second case - the value went down:
"Mashach mimenu ma'aser bishtayim v'lo hispik lifdoto ad she'amad b'sela" - he acquired the produce through meshichah when it was worth two sela, but he did not manage to hand over the money before its value dropped and stood at one sela. As far as the monetary obligation is concerned he owes two sela, while the redemption required is only one sela.
Therefore "nosein lo sela min hachullin" - a chullin coin that is exchanged for the Ma'aser Sheni produce and becomes Ma'aser Sheni money in the hands of the recipient. "V'sela shel ma'aser sheni shelo" - the second sela he may pay from Ma'aser Sheni money that was already in his possession.
At first glance this is difficult: after all, one does not redeem Ma'aser Sheni produce with Ma'aser Sheni money. The answer is that the second sela does not serve as redemption at all - the entire redemption is accomplished with the first sela, while the second is merely the payment of a monetary debt. Therefore he may pay it from his own Ma'aser Sheni money.
When the seller is an am ha'aretz:
"Im hayah am ha'aretz - nosein lo mid'mai" - one does not give definite Ma'aser Sheni money to an am ha'aretz, lest he not treat it properly. Therefore the money should be given from Ma'aser Sheni of demai, and not from definite Ma'aser Sheni money.
There are other versions of the Mishnah, according to which when the seller is an am ha'aretz he may not give him Ma'aser Sheni money at all, and both sela must come from his ordinary chullin money.
In summary: This Mishnah teaches that in the redemption of Ma'aser Sheni through an exchange one must distinguish between two points in time: the monetary obligation is determined by the value of the produce at the time of the meshichah, and the amount of the redemption is determined by its value at the time the money is handed over. If the value went up - he gives one sela, gains a sela, and the second Ma'aser Sheni coin remains his. If the value went down - he gives one sela from chullin for the redemption, and the second, which is not a redemption but the payment of a debt, he may pay from his own Ma'aser Sheni money; and if the seller is an am ha'aretz, he gives him from demai money, and according to another version - from chullin alone.