Maaser Sheni, Chapter Four, Mishnah Six
This mishnah deals with cases where there were fluctuations in the value of the maaser sheni produce from the time it was given to a buyer until the time it was redeemed.
1) If one took possession from the owner of maaser sheni for a sela, but before he had time to redeem it, it stood at the price of two selas, he may give him one sela and make a profit of one sela and the maaser sheni remains his.
2) If he took possession from the owner of maaser sheni for two selas, but before he had time to redeem it, it stood at the price of one sela, he may give him one sela out of hullin [money] and one sela of his maaser sheni money.
3) If the owner was an am haaretz, he should give him out of [maaser sheni of] demai.
Section one: A person took produce from another person in order to buy them and the money would become maaser sheni, which the seller would then bring to Jerusalem and use there. When the purchaser took possession of the maaser sheni produce, it was worth a sela, but by the time he gave the sela coin to the seller, the price had risen to two selas. The purchaser may now give the seller a sela, to compensate him for the value of the produce when he acquired it. The seller will then have to take this sela to Jerusalem and use it there. The buyer profits the second sela and the extra sela of maaser sheni belongs to him. The buyer will have to redeem this second sela of maaser sheni and bring the money to Jerusalem and use it there.
Section two: In this case, he bought two selas worth of maaser sheni produce and now the value has gone down to one sela. The buyer is liable to pay two selas. However, he can pay back the seller one of the selas with hullin, non-sacred money. He can pay back the second sela with his own maaser sheni money, which the seller will then have to bring to Jerusalem. The main point is that on the one hand, he has to pay back two selas, because that is the value of the produce when he bought it. On the other hand, there needs to be only one sela of maaser sheni, because that is the current value of the produce.
Section three: If the seller was an am haaretz, one cant give him actually maaser sheni money, because he may not take it to Jerusalem. Therefore, he should give him maaser sheni money whose source was demai (doubtfully tithed produce). If the am haaretz doesnt take it to Jerusalem,