Once an animal has been set aside as a specific korban, it must be brought as that sacrifice and cannot be redeemed or switched for another. This Biblical law (see Vayikra 27:10) includes a penalty for trying to do so. According to the Torah, if someone does try to switch a sanctified animal and exchange it for an ordinary one, both animals will become sanctified. The first remains in its original state, since it cannot be switched, and the second, by means of the laws of temura – “switching” – becomes sanctified, as well.
The Gemara on today’s daf quotes a Mishna from Massekhet Temura (25b) that discusses someone who has two sanctified animals before him, an olah – a burnt-offering – together with a shelamim – a peace-offering – and wants to switch with another animal. According to the Mishna, Rabbi Meir says that in such a case, where a single animal is positioned to replace one of the sanctified animals, and the person says “this animal should switch the olah, should switch the shelamim,” we accept his first statement and the animal becomes an olah. Rabbi Yosei says that if he really intended for the animal to switch both, recognizing the fact that a person cannot say both things simultaneously we give the new animal the status of half-olah, half-shelamim. Since such an animal cannot be sacrificed, we wait until it develops a blemish that would render it unfit for sacrifice. At that time, when halakha allows it to be redeemed, we will require that the owner redeem it and with half of the proceeds purchase an olah and with the other half purchase a shelamim. If, however, he changed his mind when he made his second statement (“this animal should switch the shelamim“), then only the first statement is meaningful, and the animal will become an olah.
Tosafot point out that when Rabbi Yosei’s opinion is discussed in Massekhet Bava Kamma (73b) the Gemara concludes that we are not concerned with the subjective question of whether or not the person changed his mind, rather with the objective question of tokh kedei dibbur – were the statements made within the same statement. If the two statements were made virtually simultaneously, then they are considered to be the same statement. If, however, there was even a small time lapse between his statements, only the first one is significant, and the second neither replaces nor affects the one that was made first.