Isn't every conversation with the US president a form of quid-pro-quo?
Possibly, though it depends on what's in the quid pro quo if it matters. In the case of withholding congressional approved security assistance until the Ukrainians investigate a domestic political opponent, there are two things wrong:
It's not up to the president to withhold the assistance.1
The president isn't supposed to use his office to go after his political opponents. Furthermore, it's exactly contrary to the foreign policy the US is trying to promote in Ukraine (one without political investigations).
Or, as we heard on the first day of the public impeachment hearings (as transcribed by rev.com):
Daniel Goldman: (04:16)
Mr. Ken, is pressuring Ukraine to conduct what I believe you’ve called political investigations, a part of US foreign policy to promote the rule of law in Ukraine and around the world?
Mr. Kent: (04:29)
It is not.
Daniel Goldman: (04:31)
Is it in the national interest of the United States?
Mr. Kent: (04:34)
In my opinion, it is not.
Daniel Goldman: (04:36)
Mr. Kent: (04:37)
Because our policies, particularly in promoting the rule of law, are designed to help countries, and in Eastern Europe and central Europe, that is overcoming the legacy of communism. In the communist system in particular, the prosecutor general office was used to suppress and persecute citizens, not promote the rule of law. So in helping these countries reach their own aspirations to join the Western community of nations and live lives of dignity, helping them have the rule of law with strong institutions is the purpose of our policy.
Daniel Goldman: (05:13)
So in other words, it is a purpose of our foreign policy to encourage foreign nations to refrain from conducting political investigations. Is that right?
Mr. Kent: (05:24)
Correct. And in fact, as a matter of policy not of programming, we oftentimes raise our concerns usually in private with countries that we feel are engaged in selective political prosecution and persecution of their opponents.
In conclusion, it's not so much that foreign policy doesn't or shouldn't rely on quid pro quo, because it does. The problem is the type of quid pro quo, it should not be used to benefit (elected) official personally, it should be in the interest of their country.
1 In reply to Sjoerd's comments, it is considered unusual for OMB, the Office of Management and Budget, to block the funds in the way that this has been done. To quote from Laura Cooper's testimony before congress, "the top Pentagon official overseeing U.S. policy regarding Ukraine", testified as reported by NBC News:
"Well, I'm not an expert on the law, but in that meeting immediately deputies began to raise concerns about how this could be done in a legal fashion because there was broad understanding in the meeting that the funding — the State Department funding related to an earmark for Ukraine and that the DOD funding was specific to Ukraine security assistance. So the comments in the room at the deputies' level reflected a sense that there was not an understanding of how this could legally play out. And at that meeting the deputies agreed to look into the legalities and to look at what was possible," she said, according to the transcript.
At the next meeting with national security personnel, she said she told attendees "there were two legally available mechanisms should the President want to stop assistance" — a presidential rescission notice to Congress or for the Defense Department to do "a reprogramming action."
"But I mentioned that either way, there would need to be a notification to Congress," she said, according to the transcript.
Asked if that happened, Cooper said, "That did not occur."