You shouldn't need the key.
When you talk to the guard in Holy, he just says that the Bloody Armlet was familiar.
When I did this quest a few days ago, it simply let me walk in an open the door leading to the cellar and the gold piles. (Which made me wish I could steal some)
Server: Taurus1
Main: -Saya-
Alt: Luxray
Pets: InuYasha; Kurogane
Level: 41 ( ;; )
BF: GrimmjowJ ( <3 )