Looking into Donald Trump's net worth history, it seems that he personally has a tendency to overestimate it, not the other way around, as he does with a lot of other subjects. He's said before he was worth $10bn+, but Forbes only valued him at around $4bn at the time. His most recent Forbes valuation has him at $2.1bn, down from $3.1bn a month prior, which is probably largely down to the impact of covid. But still, even before that he has been consistently losing a lot of money. And you can't exactly prove that those figures are just inaccurate.