The story could be written by Argon - it's that good and well set - but it doesn't seem to be one of his contrary to what I remember: an englishman IIRC travels to France and ends up in possession of the uniform of someone who used to be in Napoleon's army. The buttons of the uniform contain gems that are, of course, worth a fortune though he is unaware of this.
He ends up, I believe, romancing a relative of the dead soldier and they live in her (?) castle in France for a time before she is killed by soldiers (?) looking for the gems.
Does anyone have any idea of what I'm talking about?