What ticket do you hold and what was the price?
Anytime and off-peak return tickets on this route allow you to start short.
If it is an advance ticket you must use complete the entire journey and use the exact trains that were reserved.
In Scotland you can buy full-price tickets on the train. Which means that you can get on a train with an invalid ticket and ask the conductor for permission to travel. If refused, you can just pay again. However, there are ticket barriers at Inverness so you may not be able to get on board the train to ask for permission.