What are the consequences if, after bedikat chametz, one recited the daytime bitul chametz? Answer:
We must start with a look at the purpose and mechanism of the various bituls.
The Torah forbids possession of chametz on Pesach (Shemot 13:7) and mandates its removal before Pesach (Shemot 12:15). To facilitate this, we search for chametz the night before Pesach and physically "destroy" the leftovers the next day (bi’ur chametz). The gemara (Pesachim 6b) says that after bedikat chametz, the Rabbis instituted bitul chametz. It explains that this is out of concern that he might find some tasty chametz on Pesach, which without bitul would cause him a problem (there are different explanations on how). The gemara refers to bitul chametz at night.
The gemara does not mention bitul’s text, and slightly varied versions exist. The consensus, though, is that it applies to chametz that is unknown to the declarer at the time of bitul. One reason to exclude known chametz is because some is slated for eating during the next half day, making a statement that his chametz is worthless and ownerless disingenuous. Regarding chametz slated for burning, we want it in our possession because the complete fulfillment of bi’ur chametz is with one’s own chametz (Mishna Berura 434:7). (This is only a hiddur. We also do bi’ur chametz at a time and in a manner in which it is anyway not clear that the burning of the chametz is a special fulfillment (this is beyond our scope – see Dirshu 445:4).)
Daytime bitul is a post-Talmudic minhag designed to deal with the possibility that some of that which was purposely left over was neither eaten nor destroyed (ibid. 11). According to most poskim, it is done after bi’ur chametz and, therefore, is done with catch-all terminology that even includes chametz thrown into the fire but insufficiently burnt (Da’at Torah 434:3). If one uses the daytime text at night, it will ostensibly have applied to even that which he plans to eat and that he plans to burn. Is that a problem? Well, what does bitul do?
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According to Tosafot (Pesachim 4b), bitul makes chametz hefker (ownerless). Assuming that no one hears the mistaken declaration and takes still desired chametz, the owner can eat the nullified chametz as is or reacquire it (if it is in his house, he requires no action to reacquire it). Regarding wanting to burn his own chametz as well, he can easily reacquire some (which suffices) or all. The potentially more serious issue is the bitul’s impact on one’s mechirat chametz, which rabbis do for us the next morning. However, the same answers probably apply.
There is also a more fundamental factor. If one reads the words of bitul and does not understand their content, the bitul is ineffective (Mishna Berura 434:9). This is probably the case for one who reads the morning instead of the night version. Even if he understood the words and forgot that it was the wrong time to do an all-inclusive hefker, we should apply the concept that hefker done by mistake is ineffective (Tosafot, Pesachim 57a).
According to Rashi (Pesachim 4b), bitul is not based on hefker but is a special "mental destruction" of chametz, which the Torah indicated is significant in regard to one’s chametz. This certainly does not affect one’s ability to eat chametz he desires before the time of bi’ur chametz. It probably also does not impact the ability to sell chametz to a non-Jew. To the contrary, if anything, it is likely that the act of sale, in regard to food that he put aside in special places for that purpose, may undo such bitul for the following reason. If you do not value the chametz, how are you able to sell it? But you will be redoing the bitul the next morning anyway. Regarding the value of bi’ur chametz after such a bitul, it could in theory be negatively impactful (well beyond our scope). However, again fundamentally, bitul without intent or probably even by mistake is not valid (see Ran, Peaschim 1a).
While most likely unnecessary, it does not hurt to state that he reverses his declaration regarding chametz he is aware of.