What is the punishment for adultery that occurred between a fiancé and his fiancée? And does God prevent marriage because of it? And can the fiancé return after the engagement is broken?

Adultery is one of the most heinous sins, and Allah has forbidden even approaching it because it is an obscenity and an evil way. One of the reasons for falling into it is leniency in dealing with the suitor, as he is a stranger with whom private seclusion (khalwa) is not permissible, nor is touching his body, nor engaging in excessive conversation with him without need. What occurred between you two, in terms of adultery, is a great abomination. However, if you both repent with sincere repentance, Allah accepts repentance, forgives, and pardons, no matter how grave the sin. Repentance erases what came before it. Repentance consists of abandoning the sin, regretting having committed it, resolving not to return to it, and avoiding the causes of disobedience. If the suitor repents with sincere repentance and returns to you, there is no impediment to your marriage.