In years 1,2,4 and 5 of the shemita cycle we separate maaser sheni from all produce grown in Israel. This food is eaten in Jerusalem in a state of ritual purity. If a person is unable to transport the fruit and vegetables to Jerusalem (because they live too far away or because there is no Beit HaMikdash) they redeem the produce on a coin, and later use the coin to purchase food in Jerusalem.
The Torah tells us that:
At the end of every three years, even in the same year, thou shalt bring forth all the tithe of thine increase, and shall lay it up within thy gates.(Devarim 14:28)
The Yerushalmi learns from this that before the last day of Pesach in teh 4th and 7th years of the shemitta cycle a person must destroy any remaining maaser sheni money that they have not used:
מקצה שלש שנים תוציא את כל מעשר תבואתך בשנה ההיא" (דברים יד כח). יכול מראש השנה של שלישית? - תלמוד לומר "מקצה" - יכול בראש השנה של רביעית? - תלמוד לומר "כי תכלה לעשר את כל מעשר תבואתך בשנה השלישית" (דברים כו, יב). - יכול אפילו בחנוכה? - הרי אני דין: נמצא כאן "מקצה", ונאמר להלן "קץ": "מקץ שבע שנים במועד שנת השמיטה בחג הסוכות" (דברים לא). מה "קץ" האמור להן רגל, אף "קץ" האמור כאן - רגל.
- אי מה "קץ" האמור להלן חג הסוכות, אף כאן חג הסוכות? - תלמוד לומר "כי תכלה לעשר" - רגלו שכלו בו מעשרות, הוי אומר: זה פסח. מכאן אמרו: ערב יום טוב האחרון של פסח של רביעית ושל שביעית היה ביעור. ברביעית מפני מעשר עני שבשלישית; ובשביעית מפני מעשר עני שבשישית.
Summary: Any unused maaser sheni must be destroyed before the last day of Pesach in 4th and 7th years. (The Bavli seems to be of the opinion that it must be destroyed before the first day of Pesach, but Rambam rules like the Yerushalmi).
The Torah also tell us of a 'confession' that we are to make when brining the first fruits to Jerusalem each year:
then thou shalt say before the LORD thy God: 'I have destroyed the hallowed things out of my house(Devarim 26:13)
Here are some links in Hebrew about the mitzvah of Bi'ur Maaser:
Even though today we do not have a Beit HaMikdash, and cannot bring bikkurim, there is still a mitzvah to destroy the maaser coin.
The technical problem nowadays is 'How do I destroy my coin?' I remember one time (many years ago) spending a very long time with a hammer trying to make the coin unusable (without much success - though perhaps that is because I am a wimp). Someone suggested using a drill to make a hole in it (never tried - sounds too dangerous). Unfortuately halacha does not permit exchanging the coin for paper money (which would make things simple)
Of course the simple answer is to follow the advice of Chazal and throw it into the Dead Sea. However for most people that is not so practical.
HOWEVER tomorrow I am going to the Dead Sea on a tiyul. Tomorrow (for the purposes of this blog) is Monday, second day of Chol HaMoed (in Israel - first day Chol HaMoed in Chutz La'Aretz). I am going to (bli neder) throw my maaser coin into the Dead Sea. And because you can transfer the kedusha of the maaser from one coin to another, I will gladly add the maaser from your coin to mine, if you add your name in the comments to this blog. Then your coin at home will revert to being a regular, ordinary coin, and your maaser will have been destroyed in the Dead Sea as per Chazal's suggestion.
So, add your name to the comments (but quickly, because I'm not going back again - though the truth is that I know someone who will be there for the whole week, so if there is more demand than supply I may be able to arrange for another 'coin throwing at the end of the week).
The only problem is that someone told me that it is illegal to destroy Israeli currency. So if you work for the Bank of Israel (or Police or whoever may enforce such a law - which I can't find in a quick google search, so it may not be true) please don't read this blog, and please don't come to the Dead Sea tomorrow. Thanks.
(I normally do try my best to keep the laws of Israel, but I'm hoping that they make allowances at this time for the fulfillment of this mitzvah.)