The verse at the end of Sefer Vayikra that teaches the law of temura (see 27:9-10) makes clear that in the event that someone planned to exchange an animal that was sanctified for sacrifice on the altar in the Temple with an ordinary animal, that both will become holy. While the previous perek dealt with the laws relating to the animal that was sanctified in this manner, one sacrifice that was not discussed was a korban ḥatat – a sin offering – whose rules and regulations are unique. The fourth perek of Massekhet Temura is dedicated to discussing the special principles of the “five sin offerings that die.”
According to the Mishna on today’s daf,
If one set aside a sin offering and it was lost and he offered another instead of it, if then the first animal is found, it is left to die. If one set aside money for his sin offering and it was lost and he offered a sin offering instead of it, if then the money was found, it goes to Yam HaMelaḥ.
The reason that this animal is left to die is because it falls into the category of a sacrifice “whose owners have obtained atonement through another.” Since a korban ḥatat is sanctified as atonement for a specific sin, in the event that the atonement is attained the sacrifice cannot be brought for some other purpose. Similarly, since the sacrifice would be left to die, any money that was set aside to purchase a sin offering for a specific sin also cannot be used for any other purpose and must be destroyed.
Yam HaMelaḥ that is mentioned is, in fact, the Dead Sea, which is also referred to in Talmudic texts as The Sea of Sedom. It is common for the Gemara to suggest that a forbidden object be thrown into Yam HaMelaḥ since there are no fish and therefore no fisherman who might chance across the object and take it for personal use. It is also possible that the high salt and mineral content of the Dead Sea were seen by the Sages as elements that would ruin anything thrown into it.