How can I pass the guards?

Hint 1 of 5:

First, talk to them.

Hint 2 of 5:

What do you need to learn?

Hint 3 of 5:

Listen carefully to the explanation of the local currency system!

Hint 4 of 5:

Next, what they want from you?

Hint 5 of 5:

Nothing exceptional -- just a plain bribe!