Donald Trump has absolutely zero reason to take a salary.
As CEO of multiple companies under his corporate umbrella, there is no dinner he eats, no hotel he sleeps in, no restaurant he dines at, no trip he takes, no call he makes, no anything he does that isn’t covered as a part of doing business for at least one of his businesses.
To my understanding, Trump has stepped away from his businesses to run for President. If he then bills all his campaign time to the businesses he stepped away from, I believe that would be considered fraud.
Trump appears to have put all of his businesses into some form of “blind trust” situation where his family and business associates run the businesses while he runs for President. Because he is not involved in the day-to-day operations of non-campaign business, drawing reimbursement and/or salary from the campaign is the legal and ethical thing to do.
Finally, if campaign expenses are redirected to businesses, that would appear to be a violation of campaign finance laws - undocumented and illegal contributions from said businesses.