I don't think they can force you to give up the domain. By the same token, you should probably be careful going forward on how you monetize it. Because while you're getting big eyes over how much money these people have, they have the same amount of money if you p*** them off.
I think you're making a bigger deal of how much money they have. That doesn't mean that they'll make a stupid decision or that you can gouge. More likely if you get greedy you'll simply sour the relationship, then how much are you going to get?
I would negotiate myself personally. Middlemen don't come cheap.
This is me - I'd look at that, say I'm not using the domain and make them a reasonable offer. If they offered me 5 figures, I'd probably shake hands and think, heck, everybody got a good deal. But I take the attitude that if it's a good deal for both of us, that's good enough; I don't need to squeeze every penny out of them - I'm OK if they get a good deal too.
If you want to squeeze, there's a simple rule of thumb I read in a book written by a successful entrepreneur. Simply counter offer with twice their price. They offered $15k? Go back at $30. Then have fun and don't screw it up. The worst they can do is say no - and that's on them.
In terms of valuation, getting one would be worthless. You're not concerned about market price, you're concerned about how much this specific company will pay - and a valuation won't tell you that. Actually, I'll give you a valuation. Take the company out of the picture for a second. You've sat on this domain for 10 years and have done nothing with it right? So what's it really worth? Clearly, not a whole heck of a lot. Certainly not 5 figures, probably not even 3 figures.