Customs wasn't really what was keeping him from leaving. It was the guards at the doors. He got past them at the end because Thurman gave him his coat and allowed him to go. Dixon wasn't there to countermand his orders so the guards weren't going to disobey him.

Since he was already at the lobby I don't think he had to go through customs. That's at the very beginning when you first get off the plane and he was already past that point.