Top Voted Answer
You might already have it since you gave him ten. check your bag under "important items".
Luiman04 (Expert) - 6 years ago
You have to give him 4 mini medals to get the key
condad14 - 6 years ago
If you gave him 10 medals then he gave you the key. It is worth noting the thief's key can only open locked treasure chests. The blue doors requires the Magic Key (found in Gleeba) and the Prison Bar doors require a key that you will get as part of a story event near the end of the game.
playingforfun (Expert) - 6 years ago
This question has been successfully answered and closed.