The Talmud not tries to prove that one who unwittingly uses priestly clothing for a non-sacred purpose does commit trespass, against what Rav has just said. The baraita says that once they have worn out, they are susceptible to trespass. But this is only if they have worn out. Before they are worn out, the priest does not trespass even if he wears them when not performing the service.
The issue here is trespass with shekels donated to the Temple. The first opinion holds that one does commit trespass if he uses the new shekels, those donated this year, which will be used to buy sacrifices. But one cannot commit trespass with old shekels, for they are used for Temple maintenance. In contrast, R. Meir holds that one can even commit trespass with money set aside for Temple maintenance, such as the surplus of the Chamber, the money left over after the sacrifices were purchased.
But this is surprising for R. Meir holds “the Torah was not given to angels.” These shekels will be used pay for the city wall and its towers. People will obviously use these things for secular use. So here we see that one can commit trespass even it is obvious that people will make non-sacred use of them.
Change the name of the author of this opinion and presto—no more difficulty. It is not R. Meir who holds that one can commit trespass with old shekels. It is R. Judah. Yes, the Talmud does pull this trick from time to time.
The Talmud continues to discuss whether R. Meir holds that one can trespass with an object even though we know that people will make non-sacred use of the object. The paradigm was priestly clothing. R. Meir holds that since the Torah was not given to angels, and we know that priests will use them when not serving at the altar, trespass cannot be committed with them.
The stones of the walls and towers of Jerusalem are holy but of course we can expect that people will make use of them. Nevertheless, R. Meir holds that one does commit trespass with them. Thus even in a case where we might invoke the principle that “the Torah was not given to angels” people can still commit trespass.
Again, the Talmud changes the attribution to R. Judah.
The problem is that R. Judah does not believe that the things used to build Jerusalem are sacred. If one takes a vow and says that a certain object should be “as Jerusalem” he has not dedicated it. So how can we say that people commit trespass with the stones of Jerusalem?