Extortion: the practice of obtaining something, especially money, through force or threats. synonyms: blackmail, shakedown, exaction.
Throughout his career, Donald Trump has stolen mountains of money by hiding behind the shield of expensive lawyers and technicalities in the law. He orders services and then stiffs vendors who can't afford to sue him. He ties them up in court. He reneges on agreements, cheating people on the other end of his contracts. He committed blatant fraud in promoting Trump University, then bragged when he settled because, he claimed, he settled for much less than a court might have ordered if he'd gone to trial. He steals by staying just along the civil side of the edge between civil and criminal torts. But no matter you slice it, he steals from people.
It's no surprise, then, that Trump plans to extort payment for the wall from Mexico. Technically, as president, extorting from a foreign power isn't a crime, but he'll be relying on the methods of a criminal to extort payment.
Free countries don't pay for walls to keep their own citizens from leaving. Mexicans are free to leave Mexico without government permission, just as Americans are free to leave the United States without government permission. If a nation wants a barrier to keep outsiders from entering, that nation is free to erect barriers at its own expense to enforce the immigration laws of that nation. Only a person with a criminal mentality thinks extortion from others is an acceptable way to pay the bills.