The Torah in Parashat Bechukotai (27:10) introduces the prohibition of temura, which forbids declaring the transfer of sanctity from a consecrated animal to another animal. If an animal was consecrated as a sacrifice, it is forbidden to declare the transfer of its hallowed status onto a different animal, even if the second animal is superior to the first. One who makes such a declaration is in violation of this command, and both animals are deemed consecrated and must be offered as sacrifices.
The Rambam, in the beginning of Hilkhot Temura (1:1), based on the Tosefta (1:1), makes a seemingly peculiar remark about the temura prohibition, stating that one violates it even if he pronounces the transfer on Shabbat. Apparently, there would have been reason to believe that declaring temura violates this prohibition only on weekdays. The obvious question, as posed by the Minchat Chinukh (352:12) and others, is why we would have entertained such a notion. While it is true that Halakha forbids consecrating an animal or other object on Shabbat (Shulchan Arukh, O.CX. 339:4), this prohibition was enacted by Chazal, and it is clear that if one consecrates an animal on Shabbat in violation of this enactment, the consecration is binding. Seemingly, then, declaring temura on Shabbat should be no different than declaring temura on any other day of the week. Why, then, did the Rambam (and the Tosefta) find it necessary to emphasize that one transgresses this prohibition by declaring temura even on Shabbat?
One reason, as suggested by numerous writers, is because sacrifices may not be offered on Shabbat (except, of course, those which the Torah specifically requires offering on Shabbat). The background to this answer is the Rambam’s ruling later in Hilkhot Temura (3:4) concerning the case of one who declares the transfer of the sanctity of an animal consecrated as a korban pesach onto a different animal. The Rambam writes that if this proclamation was made before the time of the offering of the korban pesach (midday on the 14th of Nissan), it is not completely effectual. Although the second animal is deemed consecrated, it may not be offered as a korban pesach, since the transfer was done before the time when the korban pesach may be offered. Therefore, the second animal must be treated as a consecrated animal, but cannot be offered as a sacrifice. (In such a case, we wait until the animal develops a disqualifying blemish, at which point it is redeemed, and thus divested of its sacred status.) The Rambam’s ruling is discussed by the Tzelach (Pesachim 96b), who explains that when one attempts temura at a time which is unsuitable for offering the sacrifice in question, the effect of the temura proclamation is limited. The second animal becomes sacred, but the full status of the first animal cannot be transferred to the second.
The Tzelach then proceeds to clarify that this is not the case when declaring temura on Shabbat. Even though sacrifices are not offered on Shabbat, he writes, a basic distinction exists between temura on Shabbat and temura of a korban pesach before the time for the sacrifice. In the latter case, the time period is inherently unsuitable for the korban pesach, which is offered only in a specific period of time. The occasion of Shabbat, by contrast, is, in and of itself, suitable for sacrifices, but the prohibition against slaughtering on Shabbat prevents us from offering sacrifices as a practical matter. As such, the Tzelach writes, pronouncing temura on Shabbat is fully effective, just like pronouncing temura on a weekday.
In light of the Tzelach’s analysis, many writers noted, we can understand why the Rambam found it necessary to clarify that one violates the temura prohibition even on Shabbat. One might have thought that since Shabbat is unfit for the offering of the sacrifice, an animal’s sanctity cannot be fully transferred to another animal on this day, and thus the prohibition is not violated if one declares such a transfer. The Rambam therefore makes a point of stating that the temura prohibition indeed applies on Shabbat, because, as the Tzelach explained, Shabbat is intrinsically suitable for sacrifices, and it is only the incidental prohibition against slaughtering that prevents us from offering sacrifices on Shabbat.