One took off challah, but the challah that was taken off become mixed back in.
May the lady do hataras nedarim and nullify her declaration of this piece of dough as “challah”? Would it retroactively make her bracha a bracha l’vatalah?
No because the bracha at the time was said correctly. There is a different question, discussed by the Steipler and R’ Shach zt”l. If someone didn’t say the bracha by mistake, may he nullify the challah in order to say the bracha now, or is it considered a bracha sheino tzriecha. Right now I don’t know where this question was spoken about.