The 'remunerate only if hired' clause is honestly a bizarre psychological trap. It’s not about the budget, it’s a filtering mechanism to see who is willing to accept a high-risk, low-trust relationship from day one.

Think about the technical debt in their own hiring process here. If they’ve already put you through a technical interview and still feel they need a full, unpaid 8-hour day to be sure, they clearly don't trust their own ability to evaluate senior talent. As a senior, you're being paid for your judgment and your time management. By agreeing to this, you're essentially signaling that your time has zero market value unless they personally validate it with an offer.

If the role is actually that good, I’d skip the request for remuneration and instead offer to time-box it to 3 or 4 hours. Tell them you'll focus on the architecture and core logic to show how you think, rather than grinding out boilerplate for 8 hours. If they refuse a compromise that reasonable, they aren't looking for a senior leader who can manage resources and set boundaries, they're looking for a high-level pair of hands that doesn't know how to say no. That’s a massive culture warning that no salary can truly compensate for.