The Constitution. Article II section I:
7: The President shall, at stated Times, receive for his Services, a Compensation, which shall neither be encreased nor diminished during the Period for which he shall have been elected, and he shall not receive within that Period any other Emolument from the United States, or any of them.Using government money to promote a private business, thereby funneling taxpayer money into Trump's pockets. Literally an impeachable offense.
The Emolument Clause needs to be enforced here. The republicans that ignore this are putting party above country.