משנה על מעשר שני 4:1

One who carries produce of maaser sheni from a place where it is expensive to a place where it is cheap, or from a place where it is cheap to a place where it is expensive, he redeems it according to the market price of the place [where he redeems it]. One who brings produce from the threshing-floor into the city, or jars of wine from the wine-press into the city, the increase in the price belongs to maaser sheni and the expenses [must be covered] from his household.

Maaser sheni may be redeemed at the lower market price: At the price at which the shopkeeper buys and not at which he sells. At the price at which the money-changer takes small change and not at the price at which he gives small change. Maaser sheni may not be redeemed in an estimated lump. If its value is known, it may be redeemed according to the evaluation of one. But if its value is not known, it must be redeemed according to the evaluation of three, as for instance in the case of wine which has formed a film, or produce which has begun to rot, or coins which have become rusty.

One may act in a deceptive manner when it comes to maaser sheni. How so? A man may say to his grown-up son or daughter, or to his Hebrew slave or female slave, “Take this money and redeem this maaser sheni for yourself.” But he may not say so to his son or daughter who are minors or to his Canaanite slave or female slave, because their hand is as his own hand.