I appreciate you might have only found you had a spare at short notice so this probably doesn't apply here. But if you contact the club to say a ticket won't be used they do have worthy causes where they can give the tickets away for nothing so they are used. Done it on a couple of occasions with our season tickets.
Although, we did have one incident a few years back where we played Norwich midweek and someone else on the weekend. Contacted the club about the weekend match to say we couldn't attend and cards didn't work for the Norwich game as they'd given tickets away for the wrong game. Stewards were really good at sorting it quickly, intercepting the other people and sorting them out with new tickets elsewhere.
Then later on, might have even been the next season, on something like world gratitude day the club sent some Boro goodies through and a thank you note through for helping out someone less fortunate by giving away the ticket. Really nice touch from the club and unexpected.
