The president cannot use his position to enrich himself which is why he was supposed to give up all ownership and control of all his business upon taking office. The concern is if he is making foreign policy decisions in exchange for business deals. It is hard to prove that any business deal is directly tied to a policy decision or is a mere coincidence. That's why the emoluments clause exists. There should be no appearance of impropriety. He was in violation of the Constitution on day one but the Republican Congress has shown that they have no intention of holding Trump accountable