R. Akiva Eiger writes in a Teshuva (3) that the issue of whether by something that actually has kedusha i.e. the klaf of a sefer Torah, we say that setting aside is significant and makes it assur even before it is used, is dependent on a machlokes in our gemara. According to Rav Yanai who says that sending down the eglah arufa to nachal eisan makes it assur, the same would be true with anything that is inherently an item of kedusha - הזמנה מילתא היא. But, according to those who say that it would only be assur by the shechita, they would have to hold that even by something which is an actual גוף הקדושה (not just תשמישי קדושה), the הזמנה would not be significant enough to create an issur.
The gemara says that according to the opinion who says that by eglah arufa the sending it down to nachal eisan makes it assur, by the birds of the metzorah the purchasing of the birds for the use of the metzorah would make it assur. Rashi explains how we can compare the purchasing of the birds to the sending of the calf to nachal eisan, we should compare the purchasing of the birds or initial taking for metzorah purposes with the purchasing or initial taking of the calf. Rashi explains that by egla arufa where there is a later act prior to shechita to be considered the designation of it and create the issur, we assume that act is what creates the issur. But by the birds where there is no later act, the purchasing or taking is considered the act that creates the issur. The difficulty with this in light of R. Akiva Eiger is that if we are really dealing with an issur of הזמנה מילתא היא, we should consider the very first act of designation to be hazmana, not the very last act before shechita such as the sending down to nachal eisan.