The club will let you return the one you're not using and they'll re-sell it to another fan. I appreciate you've lost out here in terms of circumstances, but it would be a shame for there to be an empty seat that someone else could use.
It works much better than any other solution, IMO.
If you try and select a specific seat and someone else grabs it before you do, tickets will have sold out by the time you try and select another one.
Best available automates everything for you in real time.
My biggest advice is to select 'best available' instead of trying to select a specific seat. It's the strategy I used for the Ajax bunfight and I absolutely sailed through the buying journey.