The Torah (Vayikra 27:10) teaches the rule of temura – switching animals that are set aside for sacrifices. It is forbidden to switch one animal for another, and if someone does so both animals will become subject to the laws of sacrifices.
In the Mishna on our daf, Rabbi Yehoshua reports that he has a tradition that sometimes the temura of a korban Pesah is sacrificed and sometimes it is not, but that he cannot understand the ruling. Rabbi Akiva explains it by comparing it to a korban Pesah that was misplaced and was replaced by another animal. In such a case we distinguish between two cases.
1. When the animal was found before the sacrifice was brought, we perceive the animal as having been actively “pushed aside.” At the moment when it could have been sacrificed we chose to sacrifice another instead of it. Therefore it cannot be brought as a Pesah, nor as a Shelamim (we have already learned that a korban Pesah that is not sacrificed can be brought as a Shelamim). We therefore allow it to graze until it develops a physical blemish that will make it unfit for a sacrifice. At that time it can be sold, and the proceeds will be used to purchase a korban Shelamim.
2. When the animal was found after the sacrifice was slaughtered, it can simply be brought as a Shelamim, since we do not see it as being “pushed aside” – it simply was not available for use when the korban Pesah was brought.
The same rules would apply to temura. If the second animal was introduced as a replacement before the korban Pesah was slaughtered, both of them were available at the key moment and the one that was not sacrificed as a korban cannot be brought as a Shelamim, either. It will have to be sold (after it develops a blemish) and a different animal purchased with the proceeds. If the second animal was introduced later, it will be brought as a Shelamim.
The laws of temura have an entire tractate devoted to them – Massekhet Temura. As noted above, creating a temura is forbidden, yet doing so will create a kedusha on the replacement animal parallel to that of the original korban. In cases where the sacrifice is a nedava – a voluntary offering – this does not present a problem, as both will be brought as sacrifices. In cases such as ours, or, for example, a sin-offering that cannot be brought twice, the only option is to wait until it becomes unfit to bring as a korban, when it can be sold and exchanged for another animal that will be given to the Temple.