Leave the people alone, they have taken this risk of having such a fish in their store, to ensure wealth, luck, happiness and success for the owners and their families. You should feel lucky enough you got to see one, and better yet you can go see it anytime. It's not right of you to risk their beliefs, business, and livelihood over your wants. They know what they have, so don't insult them with a measly offer. You obviously know very little about these fish and the deep beliefs that go with them. You should strongly re-consider your actions.
My $0.02