The gemara (Pesachim 4b) writes that on a d’oraysa level there is no need to do bedikas chameitz provided bitul is done. What exactly does bitul accomplish? After all, the chameitz is still there! Rashi explains that since the Torah used the term “tashbisu” and not “teva’aru”, to burn, it teaches that the mitzvah of eliminating chameitz need not involve destroying it. Bitul alone suffices to fulfill the mitzvah of tashbisu. Tosfos, however, disagrees. The mitzvah of tashbisu begins at the 6th hour of Erev Pesach, writes Tosfos, and we know that bitul is no longer effective from the 6th hour once chameitz becomes prohibited. Tosfos explains that the reason bitul works is not because it fulfills the mitzvah of tashbisu, but rather because there is no mitzvah of tashbisu on chameitz which one does not own.
The point of disagreement between Rashi and Tosfos fits very nicely into the Minchas Chinuch’s safeik (see previous post). According to Tosfos, the mitzvah of tashbisu is a chovas hagavra to destroy chameitz. That process can only be fulfilled at a certain point in time (after the 6th hour of the day) and is incumbent only upon people who own chameitz. If one does not own chameitz (e.g. one did bitul), there is no mitzvah of tashbisu. According to Rashi, tashbisu is fulfilled by achieving the state of not owning chameitz; the mitzvah is the result, not the process. Since bitul helps achieve the state of not owning chameitz, it is part of the kiyum of tashbisu. No matter how or when one destroys chameitz, if the end result of not owning chameitz on Pesach is accomplished, that fulfills tashbisu.