20% might not sound like a big deal while you're still starting – but to put it into perspective, if you work only weekdays, that means every Monday you and every single person on your team are going to be doing unpaid work for that client.
If they were to bring something valuable to the table, then maybe it'd be worth considering. But it sounds like they're just giving you vague promises of bringing unspecified amount of other business?
I think the right way to make that kind of relationship work would be to pay some finder's fee. But even them, I'd be wary of doing anything with that client – obviously there might be more than you hadn't mentioned, but from this post it seems like they're just interested in taking advantage of you. You don't want to be doing business with someone who's out to get you.
EDIT: One more thing that stood out to me. I don't want this to seem offensive, so please take it in a I-want-to-help-you -and-think-it's-important spirit: it seems like you're starting a consultancy without really having any idea where to find clients, right?
This is something that you absolutely need to address with the rest of your founders. Maybe you need to bring a good salesperson on board, maybe some of you need to learn sales, but you definitely shouldn't just go into it without any plan – otherwise you don't have a business, you have a gamble.