Sruly Hollander asked:
Question:the gemara says before that there are only three times that patur means muter.why doesnt the gemara bring this mishna here as a fourth case.
The Kollel replies:
TOSFOS on 3a (DH Bar mei'Hani) asks your question. Tosfos answers that in this case it is obvious that it is Mutar since there is no trace of anything which should be forbidden about it, and therefore it goes without saying that this Patur means Mutar. The other cases of Patur which are Mutar, however, are all cases where the Rabanan permitted something which they normally prohibited.