The complaint is here. The plaintiff is a D.C. wine bar (Cork) that competes with Trump's new businesses. Here is an excerpt that gives you the gist:
Plaintiff's claims are based on the unfair advantage that the Trump International Hotel and its dining establishments, in a building owned by the United States and operated by the Defendants, has gained from Defendant Donald J. Trump being the President of the United States. That advantage is specifically prohibited by the hotel lease between Trump Old Post Office, LLC, and the United States, as well as the laws of the District of Columbia. The effects of that unfair advantage are magnified greatly by marketing activities of the Hotel's officers and employees and the similar activities of defendant Trump, his family, and the White House staff and/or advisors. * * * on November 9, 2016, the day after Defendant Donald J. Trump was elected President of the United States, the competition between the Hotel and Cork began to favor the Hotel much more than before the election, with the result that many organizations and individuals, including citizens of nations other than the United States, substantially increased their use of the Hotel and its various facilities to the detriment of Cork. ... The reason for the increase in business for Defendants was the perception by many of the customers and prospective customers of the Hotel, substantially aided by the marketing efforts of officers and employees of the Hotel, as well as members of the family of Defendant Donald J. Trump and others associated with him, that it would be to their advantage in their dealings with President Donald J. Trump and other agencies of the United States Government if they patronized the Hotel.