All good answers but I'd come at this from a different perspective, i.e. the perspective of a product manager or product marketer.

Here's what I mean. If the prospective large customer is in a segment with multiple competitors--and the workflows and UX mockups you do can be repurposed and rebranded with the name of one of their competitors---then I'd do it on spec, i.e. for free, if it's only 20 hours.

I'd then say to the prospective client, "Hey, this is a core competency of ours and we've built these workflows for an industry segment. We'll rev share with you on the customized 'app' we spin up with no upfront cost to you."

If (or when) they freak out at that concept of you having so much "lock in" market power in relationship to your project and future iterations of the app, they'll be glad to pay you for it upfront---especially now that they think you are a domain expert in their space.

Good luck.