He goes to golf courses / Trump hotels he owns then charges taxpayers for it at more-exorbitant-than-usual rates! Trump National Golf Club Bedminster, they charged $17k per month to use a three-bedroom cottage on the property, an unusually high rent for homes in that area. Trump's company billed the gov even for days when Trump wasn't there.
$133m tax $$$ counting for golf trips in less than 4 years. He played golf on average once every 4.8 days since taking office, a retired man may not have much time to play golf.
He donated his $400k salary each year, it all evens out. Actually, he is making millions from/during his presidency, far more than his salary.
Source:
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