A check-in is the prove of a valid ticket. A check out means paying  the correct price. They belong together.

Check out, accept the 20 euro costs and contact Customer Service. They will probably correct it. A passing train conductor will sell you a ticket with 50 euros extra costs. You have to accept it, as you do not have a valid ticket.   In this case and when it is a first fine, customer service may skip it. With bad behaviour towards the train conductor probably not. So accept it, and contact Customer service afterwards.

This is a one time solution. Check in for a valid ticket, check out with the same company for paying the right price. That is how the system works. It gives hassle when you don’t.