He did Gail a hell of a favor, though. The whole Boyd family, actually.

And he got to wear Stanwyk's expensive suits, and ended up with a raise and tickets to Rio because of him. Plus, he got a heckuva adventure and a respectable newspaper article out of the whole thing!
Underhills were not generous, Fletch STOLE from them. Thankfully, Gail 'covered' it, which means it was more like a 'forced loan', but still. Quite unlawful stuff, Stanwyk's consent was not involved.

Plus he got to nail Gail Stanwyk. And don't forget Underhill's "generosity".