What is the expiation for inciting my fiancée to leave her fiancé so that I may propose to her?

If the two fiancés have become content with each other, then it is forbidden to propose to a woman who is already engaged to him, and it is forbidden to sow discord between them, because that is a type of takhbib (incitement/corruption). This includes anyone who corrupts the relationship between a fiancé and his fiancée so that he can propose to her himself, or so that someone else can propose to her, or whose intention is merely to cause mischief. And for whoever corrupted a fiancée to annul her engagement, there is no specific expiation (kaffarah) mandated by Islamic law for him, but rather, what is obligatory upon him is to repent to Allah Almighty.