Than-key-ful For Sales

Borgen sidles up to you with a twinkle in his eye. “Hey {player}, this key has been burning a hole in my pocket for ages, and I've got no idea what it's used for! I’d be truly thankful if you could take it off my hands. I'll even give you a discount since it's right after a major holiday!"

Borgen hands you a small note with the following written on it:

5,000,000 Coins

200 Totally Worthless Ancient Coins! What a steal!