After that, in the fifth verse (279), severe warning has been given to those who act contrary to this injunction. They are told if they do not abandon ربا riba, they must face a declaration of war from Allah Almighty and His Messenger. So severe is this warning that any other warning of such severity does not appear anywhere in the Qur'an in relation to any other sin, no matter how great, except kufr (disbelief), of course. The verse then closes with the words: وَإِن تُبْتُمْ فَلَكُمْ رُؤسُ أَمْوَالِكُمْ that is, 'if you repent and resolve to leave off the remaining amount of ربا riba in future, you will get your principal. Neither will you be able to do injustice to anyone by extracting more than your principal, nor will anyone be able to do injustice to you by decreasing or delaying the return of your principal.' Here the receipt of the principal has been conditioned by saying that you repent, and resolve that you will abandon ربا riba in future, and only then, you shall get your principal.
Evidently this indicates if repentance ( توبہ Taubah) was not done by resolving to give up ربا riba, receiving the principal will no more be in order. Here are the related details. Take the case of a person who just does not believe that ربا riba could be حرام haram and therefore, he does not repent and resolve that he will have nothing to do with ربا riba anymore - then this person goes out of the fold of Islam and becomes an apostate (: مرتد murtadd). The injunction governing an apostate is that his belongings go out of his possession. As a consequence, that which he has earned during the period he was a Muslim, goes to his Muslim inheritors, and that which he earns after involvement with کفر kufr is deposited in the Baytul-Mal (بیت المال : the Exchequer of an Islamic State). Therefore, should his non-repentance be because he considers ربا riba to be حلال halal , he will not be entitled to receive even his principal. And if he does not go to the limit of considering ربا riba as حلال halal but, nevertheless, in actual practice, does not stop getting involved with it and, on top of that, gangs up with his kind and stands in confrontation with the Islamic government, he then is a rebel. His belongings too, are confiscated and placed as trust in the Baytul-Mal بیت المال ، so that it could be given back to him when he repents. Perhaps, it is to point out to such details that it was said in the form of a condition وَإِن تُبْتُمْ فَلَكُمْ رُءُوسُ أَمْوَالِكُمْ which implies that, if you do not repent, even your principal will be held back.