Maaser Sheni, Chapter 1, Mishnah 6. The Mishnah deals with someone who took money of Maaser Sheni and used it to buy an animal outside Jerusalem - not within the city, but outside it. In this law the Mishnah distinguishes between one who acted unintentionally and one who acted deliberately:
Unintentionally: If he did so without realizing that the money he was using was Maaser Sheni money - the sale is void and the money returns. The reason: the one who received the money in exchange for the animal does not know that the animal has now become Maaser Sheni and that he will be required to bring it up to Jerusalem. Since he did not understand this, it is a mistaken transaction, and the sale is void.
Deliberately: If he knew that the money was Maaser Sheni money, and knew that he was transferring the sanctity of the Maaser money onto the animal - he must bring the animal up and eat it in Jerusalem. He cannot even redeem it, from the moment he redeemed the money onto it.
When there is no Beis HaMikdash:
When the Beis HaMikdash is not standing, and he has no way to bring the animal as an offering - "tikaver al yadei orah" - the animal must be buried: it is left until it dies and then buried. "Al yadei orah" means together with its hide - one may not derive benefit even from the hide; rather, both are buried together.
In summary: In this Mishnah we learned the law of one who buys an animal with Maaser Sheni money outside Jerusalem: unintentionally - the sale is void as a mistaken transaction and the money returns to its place; deliberately - the animal has become sanctified, and he must bring it up and eat it in Jerusalem, and he cannot even redeem it; and when there is no Beis HaMikdash - it is buried together with its hide, with no benefit at all, not even from the hide.