Beitza 26a - Paskening on Bechor

The gemara concludes that acc. to r' shimon there is no difference whether the bechor gets a 'mum' on yom tov or has a 'mum' from before yom tov, even if the chacham would pasken that it is a 'mum', since the pesak only took place on yom tov, it would be assur bidieved. Tosafos explains that the issue is really a muktzah issue. Since it is assur for a chacham to pasken acc. to r' shimon, it is not considered muchan and is muktzah. However, Rashi (on the mishna) says that R' Shimon would not hold of muktzah even in such a situation, therefore the reason it is assur even bidieved is because he violated the prohibition of paskening on yom tov (which is similar to either judging or tikun). The M.B. (498:52) says that Rashi's reason is not sufficient to assur bidieved, therefore the reason to assur bidieved must be bec. of muktzah. What is the peshat in Rashi? The issur to be matir a bechor on yom tov is because the pesak is not merely a revelation of torah knowledge, but rather even if it is clear to all that it is a 'mum', it is necessary for the chacham to formally pasken that it is mutar (to the exclusion of a milk-meat question) [m.b. 50]. The issur is not the revelation of knowledge that the blemish qualifies as a mum, but is rather the effect of it becoming mutar. Therefore, by chazal being mesaken that a pesak issued on yom tov is not binding, they are actually causing that there was no issur done by issuing the pesak on yom tov. Now it makes sense why it is assur even bidieved acc. to Rashi. The difficulty with this approach is that since the pesak issued on yom tov was not binding, it will need a new pesak of a chacham after yom tov, which would be somewhat of a chiddush.