You will have no trouble whatsoever. I have cancelled return trips under similar circumstances.

Simply cancel the return flight. No one will charge you or ask you a thing. The $100 charge would only be applied should you try to use the ticket's residual value later.

The back-to-back ticketing/throwaway ticketing prohibitions are not intended to criminalize legitimate changes in your plans. Many of us have, from time to time, ended up with nested tickets because our plans changed. If you read the actual provisions in the contract of carriage, these types of things are prohibited when "intended to circumvent fare rules." Throwaway ticketing/back-to-back ticketing are thoughtcrimes and would only raise an eyebrow if consistent patterns were to emerge in your travel.