Is it permissible for me to say to a person: "Forgive me"? And what is its expiation?

There is no harm in seeking pardon and forgiveness from a created being, because it is within their capability. Allah the Almighty says: "And let them pardon and overlook. Would you not like that Allah should forgive you? And Allah is Forgiving and Merciful." (Quran 24:22) And Allah the Almighty says: "But if you pardon and overlook and forgive – then indeed, Allah is Forgiving and Merciful." (Quran 64:14) And in the Hadith: "Be merciful, and you will be shown mercy. Forgive, and Allah will forgive you."