Kevin never taught Julian how to bribe, yet Julian managed to play a game Kevin had never seen before. It was public, yet so well played that Kevin was impressed.
Without a doubt, the Imperial Central Bank's private safety deposit box likely held a large sum of cash. Once Prando unlocked that box, the money would be his. As for taking bribes?
Don't be ridiculous. He simply found a key and failed to suppress his curiosity and greed. That wasn't a crime, merely a moral shortcoming. Superiors wouldn't dismiss someone over a moral lapse, especially if it could be "handled" with some dialogue. Through this subtle bribe, Julian had already won over the two influential men at their very first meeting, an impressive feat.
Had Julian been 25 or 30 years old, Kevin would have thought nothing of it. Surviving that long without being deceived often meant enough life experience. But Julian was only sixteen—where did he gain such wisdom?