Is my marriage to a girl, after another man proposed to her without consulting her, and I am better than him financially, educationally, and culturally, and then her father approved our marriage, considered a sin that requires expiation? And what is the solution for that?

It is forbidden for a Muslim to propose to a woman who has already received a proposal from his brother (in faith), unless the first suitor abandons his proposal or gives permission for another to propose. The guardian's acceptance is considered equivalent to the woman's acceptance. If this acceptance persists, then proposing to a woman already proposed to by one's brother is forbidden. If you proposed to the woman and married her before the first suitor's proposal was annulled or he gave permission, you have committed a sin for which repentance is due. Your marriage is valid according to the majority of scholars, but it is advisable to seek forgiveness from the first suitor.