If you want to convert a democracy into a dictatorship, a good place to start is by grabbing power that doesn’t belong to you. The reasonable people in the room may be too confused to effectively stop you.
The Constitution of the United States is very explicit about impoundment of funds allocated by Congress. The President does not have the power to simply refuse to spend that money. Those funds do not belong to the executive branch. They belong to Congress.
Here’s an analogy: Suppose you put money into a bank. You know that it’s your money, not the bank’s money. But suppose the bank says “Sorry, but we’d prefer to keep this money. Go away.” Would you be ok with that?
That’s effectively what the January 27 memo is saying. Somebody is claiming ownership of something that belongs to someone else.
I believe the term for that is “criminal activity.”