441. United Jewish Appeal: The obligation to redeem disqualified sanctified animals
To your heart’s content you shall slaughter and eat meat…the ritually impure and pure… (Deuteronomy 12:15)
You know how every Jewish organization you've ever encountered sends you an envelope two or three times a year? That's nothing new. Fundraising dates back to the building of the Mishkan in the wilderness and people have been donating ever since. While people might donate money to the Temple, it was also common to sanctify animals to be used as sacrifices. Once designated as such, these animals were called "hekdesh" and were consecrated for Temple use only. (See Mitzvah #353.)
Sometimes, however, an animal so designated would receive a disqualifying blemish before it could be used. (Sacrifices had to be in perfect physical condition. You couldn't offer, say, a goat with a broken leg. See Mitzvah #286.) In such a case, it was a mitzvah to redeem the animal by purchasing it with money. The money would be used to purchase a replacement animal for sacrificial use, while the original animal returned to its former, non-sanctified status and could be eaten as regular food.
The reason behind this mitzvah is God's generosity. It would be perfectly understandable that, if a sacrifice became unusable, it would simply be the donor's tough luck. However, God does not require people to absorb so great a loss. Rather, he allows them to buy back the original, disqualified animal so that they should not be subject to a great financial burden. In fact, by making it a mitzvah to redeem the disqualified animal (as opposed to simply permitted), God encourages people to do so. (Were it merely permissible, people might be reluctant, feeling that it might be inappropriate to buy back a once-sanctified animal. Now we know that God actually wants us to do it!)
This mitzvah only applies at a time when the Temple service is in effect. It is discussed in the Talmud in the tractates of Bechoros (14a-15b) and Temurah (32b-33b). It is codified in the Mishneh Torah in the first chapter of Hilchos Issurei Mizbeiach. This mitzvah is #86 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.