“The president’s interests” are not the goal of U.S. foreign policy. More important, the president is not the sole author of U.S. policy. In the case of Donald Trump and Ukraine, these two facts ought to be obvious, because the whole scandal is about the president arbitrarily withholding aid to Ukraine that had been approved by the U.S. Congress.
The president takes the lead in the implementation of U.S. foreign policy—but as with his other responsibilities, he does so in partnership with Congress. The U.S. Congress influences foreign policy through its ability to grant or withhold its approval for the president’s chief foreign policy and military officials—including the secretary of State, the secretary of Defense, the National Security Advisor, and U.S. ambassadors—through its ability to grant or withhold its approval of treaties, and through the power of the purse, which allows Congress to mandate financial support for some countries or programs while withdrawing it from others.
The president doesn’t make foreign policy by himself, nor does he make it for himself.