What is the expiation for a void contract whose parties cannot be reached?

If the woman is from the People of the Book, it is not permissible for her to assume responsibility for her marriage contract, because a Muslim cannot be the guardian of a disbeliever. Ibn Qudamah said: "A Muslim has no guardianship over a disbelieving woman," based on the Almighty's saying: "And those who disbelieved are allies of one another." Accordingly, the aforementioned marriage is invalid and must be annulled. If they wish to renew it, it must be in the presence of her guardian or his representative. The children are attributed to the husband if he believed the marriage to be valid, for Muslims are in agreement that if a husband believed a marriage to be permissible when he engaged in intercourse therein, his child is attributed to him and they inherit from each other. The questioner should inform the spouses of the invalidity of the marriage if he is able; if he is unable, that obligation is waived from him. There is no expiation for this act except repentance and seeking forgiveness. It is not a condition for the marriage formula to include the specific wording of 'marriage' or 'giving in marriage' according to some scholars.