Can he though?
He might never have been a billionaire at any point, he's always been leveraged to the gills and bleeding money. Plus the fraud, making up fake names to call up Forbes to be declared a billionaire, and the fact he is constantly hustling (while campaigning and terminally old) for a few more million
Most of all, there's the fact that billionaires are only billionaires on paper. Their wealth would shrink massively if they can't time the sell-offs at the right time, most commonly known billionaires could keep the title selling for 10 cents on the dollar, but Trump isn't near that level no matter how you slice it
Mar a Lago is estimated at like 200-300m, his apartment is worth tens of millions, and pretty much everything else he only owns a partial stake in and is likely leveraged even if it could be cleanly sold. And who knows what other debt factors in - he's had a lot of failed projects and a lot of unpaid invoices
None of them are proof, but altogether it doesn't paint the picture of a billionaire - a billion dollars is a truly unthinkable amount of money, and I can't see a showoff like him not collecting things to brag about
I don't think he has it, or if he does it'd be a close thing
And to leave you with a final thought - Trump owns 3 guns, one of which he risked his freedom to hide away. What kind of billionaire both cares about guns enough to break parole (even if the prison risk was very unlikely), but also doesn't have a small armory professionally decorated just to show off?
That was due to additives to ethanol cleaning agents - it was a thing done on purpose