Dear Rabbi Kornfeld Shlita, How have you explained the above mentioned sugya? Rashi is unclear as to what the possible Issur may be. In his first comment he mentions that we are not discussing the problem of the donkey going out to Rshus Harabim only with the donkey becoming chilly and sick. On the other hand in DH, Mirdaas Mutar, Rashi explains that since the donkey will have Tzar if it does not have the Mirdaas on, the Mirdaas is Orchay and Darko Bkah ans susequently not considered a Masoy. This unclear since Masoy is a term referring to Mleches Hotzaa Mrshus Lrshus. Is there a contradiction in the way Rashi explains this Sugya?
A big Yassar Koach on your intensive work for Har batzas Tora.
Rav Kibbutz hafetz Chayim
Your question is indeed an excellent one. It is asked by the Rashash, as well as by Rebbi Akiva Eiger, who adds that Rashi's words are even harder to understand when we consider that the Mirda'as is a Masuy if not placed on the donkey from before Shabbos, as the Gemara said earlier on
the page (according to Rashi 54b DH k'd'Amran, see Tosfos here DH v'Hu).
Rashash adds that if the reasoning has to do with Masuy, why is it then prohibited to remove the Ukaf on Shabbos?
Besides, we may point out that Rashi throughout the Sugya tells us that placing a Mirda'as on a donkey is Mutar because of Tza'ar (cf. Rashi 53b DH u'Mishum Tza'ar).
I haven't found an answer to your question.