Naughty Guards
Arnet looked tensed seeing that the boy was not lured into his trap. He still did not know who Alex was and thought the greedy boy came here just for the contents of the ring.
He thought maybe the boy would swear the devil's oath easily and since the boy was able to come here without restriction from the guards and even talk like that with Joseph then he should be the son or relative of whoever had defeated them in the battle.
After the boy takes the devil's oath to release him after he tells him what's inside the ring then he can easily get out of here as no one would want to go back on the Devils oath. Even the relative of the boy would never want that. But it seems like the boy was not stupid nor he was greedy.