משנה על סנהדרין 3:9

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.