Facing the similarly-sized Blue Dragon and Red Dragon, fighting two against one wasn't a good idea, and the Brass Dragon's choice wasn't to stand firm against Colorful Dragons and Metal Dragons as mortal enemies, but rather to spend money to avoid disaster.
After intimidating the Brass Dragon a bit, Leon still chose to accept the Brass Dragon's proposal.
The Blue Dragon's thought was that this place was, after all, the Brass Dragon's lair, where the Brass Dragon had been operating for decades, perhaps centuries, gaining the home-ground advantage, as if with a wave of his hand he could close the great hall's doors. If a fight were really necessary, that timid dragon, once desperate, might fight to the death, resulting in a pyrrhic victory with no real winner.
In short, after some haggling, the Brass Dragon opened his treasury, which had a rather pitiful collection, and allowed Leon and Tassera to select treasures.
