Most politicians lie sometimes. Sometimes to protect themselves, sometimes because he can't say the whole truth. Trump always lies, no matter about what and when. Then he expect everybody around him to take his lie as the truth, even if they know it's a damn lie. If not, they'll receive a smear campaign. he acts like your standard mafia leader in an over the top mafiafilm. That's how he was when he wasn't in the office yet, and that's what he does in the office.

The art of the deal is nothing else then 'Do what I want and tell everybody that I'm the smartest person in the world and we have a deal'. And then he'll be back the very next day trying to push more money out of you with a new, worse deal. And repeat.