Maaser Sheni, Chapter One, Mishnah Five
1) One who bought water or salt, or produce still joined to the soil, or produce which cannot reach Jerusalem, he has not purchased maaser [sheni].
2) One who bought produce unwittingly, the money must be restored to its former place.
3) But if intentionally, the produce must be taken up and be consumed in the [holy] place, and when there is no Temple, it must be left to rot.
Section one: Water and salt are not considered to be food and therefore one should not buy them with maaser sheni money. Produce still attached to the ground also is not considered food and therefore, it too should not be bought with maaser sheni money. Finally, one should not buy produce that will rot before it can reach Jerusalem. In all of these cases, that which he bought is not considered maaser. What he will need to do is use an equivalent amount of money to buy the type of food that he should have bought in the first place. That food he should bring to Jerusalem and eat there.
Section two: This person bought produce using maaser sheni money without knowing that it was maaser sheni money. The result of this would be that he would have to take the produce to Jerusalem and eat it there. However, since he did this by accident, the purchase is considered an accidental purchase and he can return the produce to the seller and get his money back. Since it was accidental, we dont trouble him to take the food all the way to Jerusalem, which might be difficult.
Section three: Maaser sheni money should be used to purchase food only in Jerusalem. If a person nevertheless intentionally buys produce with maaser sheni money outside of Jerusalem, he has to take the produce to Jerusalem and eat it there. He cannot ask for his money back. If there is no Temple, then he must let the produce rotit is forbidden for him to redeem it for money.