The Gemara will often begin a sugya with an initial understanding (havah aminah) of a mishnah or halacha and as the Gemara continues will change until it reaches the maskanah. Often, the maskana will be different from the hava aminah but not entirely. The maskanah can change a specific premise that was proposed initially but not deny every premise that was proposed.
I have seen poskim explain their understanding of the Gemara and what premise(s) was ultimately rejected in order to paskin based off of the initial understanding's premises that were NOT rejected.
I remember seeing this done based off of premises explicitly presented by the havah amina that were not rejected by the maskanah.
My question is: Has a posek ever given a psak based off of a premise presented in the havah aminah that was not explicitly presented by the gemara (or a Rishon)?