Fafnir did try to use some of his gold to pay for housing when they were house-hunting. It got rejected.
Apparently, gold exchange is hard in that world.
An unbelievable idiot of a real estate agent.
As much as dragons love their treasures, I don't see why they couldn't use them for their own benefit. Normally there just aren't any need or even a way for them to do it.
He probably don't have access to a no-question-asked pawn shop. One huge lump of gold certainly don't just came out of nowhere.
It's like deciding to steal money left in the toilet or not. There's "too small to bother", "risk low enough, take it", and "too much, looks dangerous, better leave it alone"