I have always wondered why this would be the only thing he doesn't lie about. It doesn't make sense to me. My guess is that he donates, and writes off his salary, to a charity he controls. Then the charity uses the money to get more portraits of himself painted. They are painted in a Taiwanese factory. The charity gives these beautiful portraits of Donald away to their donors as a thank you. The donors happen to be Trump's "for profit" Gulf Clubs. The clubs write of the donations and hang the portraits in their club houses. I'm not serious, but he has done similar and got caught. Does he ever say what charities his salary is donated to? I don't don't think so.
As I'm sure you know, when investing in real estate, it is not only common, it is usual, to show net losses the first few years, especially if you are allowed to take accelerated depreciation on top of what are nearly always big renovation expenses and big expenses associated with the acquisition. That said, the IRS has a rule, not well enforced, that a legitimate business must eventually turn a profit, and they have guidelines such as a profit so many years out of so many. If one cheats on taxes and attracts the attention of the IRS, as Trump will do if he doesn't win and a new administration gets rid of Trump's hand picked IRS head, then these guidelines will rear up and bite Trump in the ass. Trump is continually buying new properties and these create losses for him, but what about the properties he's owned for more than a decade. Those are the ones that had better be showing an occasional profit, otherwise the IRS can view them as not legitimate businesses.
Given Trump's veracity record, my guess is that he will be found, on close examination, by green eye-shaded tax examiners, to be a monumental tax cheat, lying about almost every number on his returns! This behavior he comes by legitimately: his Dad escaped going to prison during the Eisenhower era by less than the hair on his teeth.
Any reasonable person should be asking, why would Trump's tax returns be the only thing he doesn't lie about? The answer to this question won't send him to jail, of course, but it will produce a very thorough audit at some point soon.
