What if the rabbi informs you on Yom Tov or Chol ha-Moed that he forgot to sell your chametz to the goy on Erev Pesach. Does the bittul help? Can one take the rabbit to din Torah or is there only tar’umos?
If the rabbi forgot to sell the chametz, and you are informed of this on Pesach, there is an obligation to destroy the chametz on Pesach.
The rabbi can be requested to compensate for the loss.
[Concerning after Pesach (in the event that the matter was only discovered after Pesach), authorities dispute if the nullification (bittul) alone is sufficient to permit the chametz after Pesach, where the person in question was unable to fulfill the biur. See Biur Halachah (448), who rules that the chametz will be permitted in its benefit (but not in its consumption), and see Aruch Ha-Shulchan (448) who writes that the chametz will be entirely permitted in this case.