I'm certain that Trump hires lawyers on a contingent fee basis for certain kinds of cases. He wants results and a contingent fee is the only way to assure that payment is made only if results are achieved.
Plus, I think I read earlier in the thread that it was a property tax assessment case. Those are cases where the lawyer is hired to challenge the tax assessment for a property. They are almost always handled as contingency cases. (I have been on both the owner and attorney sides of those cases.) Essentially, the owner promises the attorney a percentage (usually 1/3rd) of whatever property tax savings result from a successful reduction of the tax assessment for the property. These deals are nice from the owner's perspective because the owner takes no risk.
A possible scenario: Trump had a contingent fee contract with these lawyers for a tax assessment appeal case. The attorneys spent a lot of time working on it, but were not successful. They figure Trump is a rich guy so he ought to pay them for their time anyway. Trump says no because that was not their deal. There would be nothing unethical about this, and is exactly the kind of guy I would want as president.
Your observations make sense to me.
Especially given the fact that lawyers know how to get paid when payment is due.
My attorney had a personal lien against a notorious racketeer for years. Renewed it every 10 years, as I recall.