Interesting aside: what are the legal consequences of donating to an organization that is branded a "terrorist organization"? Can you be punished retroactively?
Yes. These lawless people will do anything they want under color of law. Do not expect rationality or good faith from them. Expect savagery.
[Note: I'll interpret the down-votes as disagreement. I hope you're right. But I think donation require courage. Just make sure you can withstand an audit. I don't think you have to worry about orange jump suits yet, Peter King notwithstanding.]