It sounds like getting rid of him is the right decision. Your goal should be to find him a suitable home. If you really "love this fish" I don't think you would have written, "If you're interested make me an offer I can't refuse and we can go from there, but if our numbers don't match up, then I'm set on keeping him. Money talks, ya know?"
Numbers should not matter. You can buy these guys for $20 these days as babies and within a short time they will be as large or larger than yours. If you really cared about the fish, you would be more interested in finding someone who can care for him as opposed to trying to get top dollar for him.
I apologize for being crass, but you made the mistake of buying a fish you can't care for, so if you have to donate him in order to get him to a suitable home, you should take the hit and just give him away. Give him to TBTH if he has an appropriate setup. Wouldn't you feel better knowing he went to someone who is going to properly care for him as opposed to some chump with a 100G that happens to pony up $125?
Make the responsible decision.
