Continued from Parts (1) and (II)
R’ Akiva Eiger answers his question on the Ran by contrasting bittul with other forms of kinyanim. The rule of devarim sheb’lev means that mental reservations have no legal impact on an act of a sale – actions speaks louder than words. However, bittul is itself fundamentally is a mental renunciation of ownership – the verbal declaration is just a means of announcing the change of status our decision has effected. The Ran later writes that bittul is actually even weaker than hefker. Since chamietz is assur b’hana’ah it is inherenly ownerless, but the Torah penalizes someone for keeping it in their possesson – bittul simply negates that penalty from setting in, but is not itself what imposes the status of hefker on the chameitz. Devarim sheb’lev cannot undo action – but they perhaps can undo other devarim sheb’lev and render the original bittul invalid.
R’ Akiva Eiger quotes a second answer from his son in law. Even though bittul suffices to remove the issue bal yera’eh and cannot be undone, if one finds chameitz and delays destroying it, one has violated the positive mitzvah of tashbisu, which commands one to destroy chameitz found in his possession.
This implicit assumption of this answer is that tasbisu is not just an issur aseh, a commandment to passively avoid owning chameitz, but rather demands that one actively and immediately destroy chamietz. This issue is a major debate in Rishonim and Achronim, with many consequences. See Minchas Chinuch, and maybe more later…