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...
The Supreme Court will jiggle the constitution and some forgotten legal precedent from the magna carta will fall out and 5-4 this is actually not a problem at all if you think about it.
Everything Trump does, is, by definition, a minor question.