What is the expiation for repeatedly falling into sin between an engaged couple after repentance and seeking forgiveness, and is fasting or charity sufficient for that?

If the two betrothed parties have not yet عقد النكاح (formalized the marriage contract), then they are considered strangers to each other (أجنبيان), and privacy between them (الخلوة) is not permissible. Whoever falls into this sin must perform sincere repentance (التوبة النصوح), which entails: abandoning the sin, regretting it, and resolving not to return to it. This repentance must occur before the soul departs the body and before the sun rises from the west. Along with repentance, one must increase in righteous deeds. They both must not repeat such an act, and they should expedite the marriage if possible. Otherwise, the woman must completely distance herself from the man.