This makes no sense to me. The dragon didn't turn up of its own accord - you (as GM) wrote it into the story! If that's acceptable, then why not the "deus ex machina bailout"? Or converesely, if you don't like the latter then why did you write in the former? To hose the paladin's player? Some other reason?
In the OP's scenario the PC talked to the dragon, and rolled well. Why did the GM apparently disregard that outcome and nevetheless have the dragon push the point? Is that some sort of anti-deus ex machina?
I don't generally do deus ex machina bailouts because I feel that doing so undermines player agency. I've played under GMs who used bailouts with regularity, and that's how it made me feel. So I don't do it when I GM.
There is one exception, which is if I screwed up. If I made a mistake that would cause permanent issues, I won't hesitate to fix the issue with a little DM magic. That's only fair, IMO.
Personally, given that the player rolled well, I wouldn't have given him the response that the OP did. Heck, given that his argument was that he was on his way to destroy an enemy of the dragon, he probably wouldn't have even had to roll at all in my game, unless maybe the dragon had reason to believe it could blow back on him.
I did actually have a very dangerous dragon in my campaign a while back. I had foreshadowed his existence multiple times in the campaign. He was the oldest class of red, and one of the oldest and most dangerous creatures in the area. The characters were seeking an artifact that a divination spell had indirectly revealed to be in his hoard. I also offered an alternative quest to steal an item from some Giants, which wasn't as powerful as the artifact, but could still get the job done. They decided to go for the dragon.
Wisely, however, they chose to negotiate. They were quite strong as a group, so I have little doubt that they could have slain him, but it would have almost certainly resulted in casualties, which they sought to avoid.
The dragon was old and nearing the end of his years. He had amassed all the treasure that a dragon could dream of, and more. The only thing he truly lacked was companionship. So he offered them a choice. They could take the artifact if they left one of their own with him. After some discussion and negotiation, one of the NPCs agreed to stay, provided he continued to collect his share of treasure.
It was certainly not the same. There was no paladin in the party, although there was a cleric. But it's the closest scenario I could recollect from one of my own games.