This is a shameless gimmick.

Trump's decision not to divest his businesses netted him $446 million in personal revenue in 2019—hundreds of thousands of which came from taxpayer-funded payments to his properties.

That's a thousand times more than the salary he "donates" https://twitter.com/realDonaldTrump/status/1294397381073678339

It's not just that he's earned hundreds of millions of dollars as president. It's that the donation of his taxpayer-funded salary is likely more than canceled out by the taxpayer money spent at his businesses by the federal government https://www.washingtonpost.com/politics/secret-service-has-paid-rates-as-high-as-650-a-night-for-rooms-at-trumps-properties/2020/02/06/7f27a7c6-3ec5-11ea-8872-5df698785a4e_story.html

Also this: "Trump received nearly $12 million for condo sales in buildings he owns and co-owns. A large portion of this sum came from anonymous LLCs. In fact, all of the $7.4 million paid to Trump for condo sales in Trump Parc were from unknown buyers." https://www.citizensforethics.org/president-trumps-2019-personal-financial-disclosure/