Yeah: If someone donates money to the government for any purpose, that becomes government funds, and the Constitution says Congress and only Congress has the authority to decide where those go. Not the President.

So either Trump is doing this officially as a President, but with funding that is illegal/unconstitutional...

... Or the funding is legal, but Trump is doing it as a private citizen, which is a federal crime. [0]

[0] https://www.justice.gov/archives/jm/criminal-resource-manual...