What is the expiation for an oath, and what must I do, knowing that I swore to a stranger by Allah and by the Qur’an that I would not marry anyone but him, then I left him and repented?

Praise be to Allah for bestowing upon you repentance and cutting off the relationship with this foreign man. You are not obliged to marry him, because if an oath is made concerning a permissible matter, the person who made it has the choice between fulfilling it or breaking it with an expiation for the oath. Breaking it may be recommended or obligatory if marrying him is disliked or forbidden (such as if the man is a transgressor or a disbeliever).