On a luxury double close, the spread becomes public record and the end buyer can read it

Both routes disclose, so the question is timing rather than concealment. Disclose the fee before the buyer is emotionally committed and it's a term. Let them find it on a settlement statement at the table and it's a grievance, and grievances at $2m get lawyers involved.

On the documents, the recording point needs a caveat you'd want in the same breath: roughly a dozen states don't make sale prices public at all, so in those the in-price isn't retrievable from records the way you're describing. That doesn't help you much, because the title company and the settlement statement still exist, and in an assignment the assignment fee sits right there.

Some operators try to move part of the compensation off the settlement statement into a separate consulting or fee agreement. Whether that's permissible depends on your state's brokerage and disclosure rules, and in several states getting paid for procuring a buyer without a license is exactly the conduct the statute targets. That's a question for a real estate attorney in the state, and it's not one to guess at on a transaction this visible.

The risk you haven't named is your seller, not your buyer. In a double close the seller can also see, or be told, what the property resold for the same day. A personal representative of an estate who learns the property moved for $110k more within hours has a fiduciary duty to the beneficiaries and a reason to make noise about it. That complaint travels further than a buyer's irritation does, because it goes to the attorney who introduced you the deal in the first place. Your source of deals and your exposure are the same relationship.