Ike disagreed with Truman and Rhee and wanted to end the war. Ike had been briefed by Truman and Acheson before he went. Truman later called his visit a “piece of demagoguery” because Ike spoke his mind while he was there and met with our military leadership.
Regarding the Logan Act, Ike didn’t negotiate with Rhee, he met with him to learn of Rhee’s views and then sent Rhee a letter with Ike’s intentions.
So there was no negotiation or any agreements, just statements of opinion from both sides. Perfectly OK.
Check out: Harry and Ike: The Partnership That Remade the Postwar World, Steve Neal, 2002.
Funny how Ted Kennedy ASKED the Russians to intervene to harm Reagan’s re-election...no Logan Act prosecution, or even investigation.