My wife made dough using 2.5 kilos of flour (in israel) and baked a small piece and ate it. afterwards she remembered that she has to take challah, and took challah on the remaining dough. I have two questions.
1) Did the hafrashas challah work retroactively on the peice she ate, or did she eat tevel?
2) Can I continue use the oven that the piece of dough that was removed before hafrashas challah was baked in?
1. Although it appears that after a person ate tevel, separating challah will not help retroactively (because what a person eats no longer exists), some write that the later hafrasha does help to retroactively remove the transgression. See Tosafos Yom Tov (Gittin 8:3), and also Mishnah Halachos 1:4.
2. The main question relates to the trays that came into contact with the dough, and not to the oven itself.
Provided hafrashah was done (within 24 hours of baking), the separation of challah will apply to the taste absorbed in the trays, and the trays can be used lechatchillah without concern. See Chazon Ish, Demai 15:9; Mishpetei Eretz 2:12.