R’ Chaim Markowitz discusses an interesting shitas haMordechi here. The Mordechai says that a blind person can be motzi others in Kiddush even according to R’ Yehudah who holds that a suma is chayav in Kiddush only m’derabbanan and others have a chiyuv d’oraysa. Just like you can be yotzei kiddush during tosefes Shabbos (which Mordechai holds is derabbanan) because it will eventually become dark where there is a chiyuv d'oraysa, so too, since the suma may potentially regain his eyesight, the present status of chiyuv derabbanan is sufficient to be motzi a d'oraysa. What kind of combination is this? If a mechuyav m'derabbanan is considered a “bar chiyuva” parallel with someone who has a chiyuv d’oraysa, then why do you need the sevara that things may change afterwards? And if the driving sevara here is the later potential for a d’oraysa obligation, then even someone who has no chiyuv at all now should be able to be motzi someone else?
R’ Elchanan Wasserman (Koveitz Shiurim II:30) writes that the Mordechai is addressing two factors that have to coincide for one to be motzi another in a mitzvah -1) being a bar chiyuva 2) being actually obligated b’poel to do the mitzvah at that moment. The potential future obligation turns one into a bar chiyuva. Yet, that is insufficient to be motzi someone if no present obligation exists. For that, the Mordechai holds at a minimum a chiyuv derabbanan is necessary.