The "I scratch your back and then you scratch mine" isn't the illegal part, it's about how it's used, and how it's dealt with.
It depends on the "pro" - Was it done for a legitimate, or a corrupt reason?
"You sleep with me and I'll get you that promotion". "I'll pay you and you make those tax issues go away."
Here's an
NYT article with 15 times Trump denied that he got anything out of the deal.
Then all of a sudden Mulvaney says Trump did get something out of it, but it wasn't a big deal.
Then he attempts to go back and say he didn't say that thing everyone heard him say.
Conditions on aid isn't a bad thing. It's again, the condition that matters.
In the Ukraine case, the aid was approved, then Trump held up the military aid to Ukraine until that country agreed to assist with an investigation into his personal political rival. Does that sound like it's in the best interest of the American people?