Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
In his response to your question, Prof. Dr. Monzer Kahf, Professor of Islamic Finance and Economics at Qatar Faculty of Islamic Studies, states:
Riba, mainly, is the interest that one gains from lending money to others.
What you did is, of course, riba. Riba is always consensual and cannot be otherwise. If it is not consensual, it is then theft or robbery!
In Islamic finance practice, they impose a high fine in order to deter the debt from any delay. But this fine is given to charity as any earning by a loan is riba and is in fact taking a part of the property of the other person without any justification, because by lending you transform your property from cash to a debt and the debt, by its own nature, does not increase.
These 30$ that you are getting must be counted toward payment of the 100$.
Allah Almighty knows best.