Is a man considered unjust if he promises a girl marriage and then leaves her, and what is the ruling on his promise and his oath, and is expiation obligatory upon him?

Failure to fulfill a promise of marriage to a girl carries no blame, especially if the impediment comes from the parents; because obedience to them takes precedence over fulfilling a recommended promise. As for the oath, you are obligated to perform its expiation, which is: feeding ten poor persons, or clothing them, or freeing a slave. If you are unable to do so, then fasting for three days. It is not permissible to resort to fasting if one is able to perform any of the first three options.