It's also not illegal to give somebody $40. If the intention is to get a few grams of weed in return then there's a problem isn't there?
If a cop finds you trading money for drugs and your argument is that it's within your rights to give somebody money, you're not fooling anybody.
The president can withhold aid, but that's not the issue is it? The issue is trading aid & a WH meeting for a political "favor"
As for corruption in Ukraine, it's ironic that Trump never said that word in either phone call to Zelensky. There's no indication that is the reason the money was held up. You'd think there'd be plenty of talking about it, especially during the phone call. Instead Trump pushed for a Biden investigation. Multiple officials independently confirmed this was the objective and agenda in separate testimonies.
It doesn't get much more definitive.