91b bottom.
R popa tried to pasken like may hava ley lemeebad rashi explains that he was mattir a woman who was muttar with edim to go back to her husband. But reb huna said that all the mishnayos says we don't pasken that way and the terutzim are dechukim.
But the gemoro said on omud A, that rav paskens like reb shimon that its muttar, and the whole shakla vtarya is only if the chachomim agree to thus concept of may hava ley lemeebad, but the halocha is clear like reb shimon!?
(This q is especially on rashi, who explains that reb popas psak was talking exactly about this kind of returning a woman who was mutar with edim)
Avrumi Hersh, London england
Shalom R' Hersh,
Yasher Koach for this insight. I see the Rif identifies your point as an indication that the Halachah indeed does not follow Rav and Rebbi Shimon, even though the Gemara seemed to portray Rav Huna/Rav as the definitive ruling using the term Hachi Hilchasa. Nimukei Yosef relates that all the Geonim also rule stringently, based on the challenge to Rav Papa.
I hope this helps!
Best wishes,
Yishai Rasowsky