Torah Temimah cites Keritot.
“And he did not know and it shall be forgiven him” — one who brings an asham talui and then learns that he never sinned: if the blood has already been thrown, the meat is still eaten.
Diyuk of “lo yada” at the time of selichah — zerikah locks the status.
Once the blood is thrown, the asham talui has completed its role even if the doubt resolves toward innocence. Forgiveness is fixed at the moment of the blood service; later knowledge that no sin occurred does not invalidate the offering.