Not sure if they're the only bank that will lend but he certainly owes them a bundle of money: https://www.bloomberg.com/news/articles/2018-11-28/mnuchin-s...

"According to an analysis by Bloomberg, Trump now owes Deutsche around $300m. He has four large mortgages, all issued by Deutsche’s private bank. The loans are guaranteed against the president’s properties: a new deluxe hotel in Washington DC’s old Post Office building, just round the corner from the White House; his Chicago tower hotel; and the Trump National Doral Miami resort."

https://www.theguardian.com/us-news/2017/feb/16/deutsche-ban...

And this: https://www.theguardian.com/business/2017/jan/31/deutsche-ba...

And this: https://www.wsj.com/articles/when-donald-trump-needs-a-loan-...

And well, you know where this all goes...