Paul wanted to shout at his father. He wanted to tell him that his marriage wasn't a business deal. How could he use this opportunity to take revenge on the Johnson's? He was taking benefit from the innocence of a teenage girl.
As much as he wanted to say those words aloud, he also knew his father well and that he wouldn't be convinced until he squeezes out the great amount of money for the loss in business he faced apparently because of his ultimate rival. The Johnson's.
The idea the he was accepting Scarlett's and his marriage was enough entertaining and any money didn't matter for that.
Despite the amount father Harrington priced for their marriage, Paul without further hesitation took the word to Scarlett's ear.