Question about the beginning.

At that (post office? Pensioner's money collecting company?) our lead character was showing off and talking about "dynamites" and that man got mad at him saying "Do you want your money or shall I call the police?" but what is that all about even?

Did the man behind the desk simply not want to talk to him and not like his jokes?

And why does he and several other workers afterwards attack him, anyone know, thanks.