Yevamos 99b - Safek Making Agent For Korban

Rashi in his first peshat says that the safek kohen safek yisroel can appoint someone to bring his korban for him as a shliach, since a kohen has the right to bring it himself whenever he wants, he can also appoint a shliach, so too this safek kohen who is entitled to bring the korban from a monetary perspective (hamotzi meichaveiro alav ha'raya), he can also appoint a shliach. Rashi asks that since practically he cannot sacrifice the korban himself since he is a safek yisroel, he should not have the ability to appoint a shliach. Tosafos answers that he is like an elderly or ill person who cannot sacrifice himself, yet he can appoint a shliach. Tosafos seems to understand that when we determine whether he can appoint a shliach, we look at whether he is technically fit to do it himself, and on the tzad that he is a kohen he is techincally fit to bring it himself, just that practically he cannot since he may be a yisroel. Therefore, he is considered fit to bring the korban, just that it is practically not possible for him to do it himself (pumei hu d'kaiv lei), so he can appoint a shliach.

The question on Tosafos is: How does Tosafos compare him to an elderly or sick person, since Tosafos says that the rationale to allow an elderly or sick person appoint a shliach is that "he is fit with difficulty to do it himself". But in our case where he is a safek yisroel and practically speaking cannot bring it himself even "al y'dai haechak", he should not be similar to an elderly or sick person? It must be the comparison to elderly or sick is not a proof, rather an explanation, that there are cases where one cannot do it, yet he can appoint a shliach, but the rationale is not the same!