The problem is that negotiation only means something if the parties trust each other to follow it (or an agreement will be enforced by external parties with credible ability to do so).

Russia and Ukraine already have an agreement - the Budapest Memorandum on Security Assurances signed in 1994. It opens with this: “The Russian Federation, the United Kingdom of Great Britain and Northern Ireland and the United States of America reaffirm their commitment to Ukraine, in accordance with the principles of the Final Act of the Conference on Security and Cooperation in Europe, to respect the independence and sovereignty and the existing borders of Ukraine”. Note that the 1994 borders of Ukraine include Crimea and the entire Donbas region.

If agreements only apply until Putin feels it is ready to take more of Ukraine, and then Putin can just take more by force, but not risk what it has ‘locked in’ through Ukraine upholding its previous agreements, then an agreement is meaningless for Ukraine - at best, all it does is lets Russia recover economically and rebuild reserves so it can prepare for a large push and take more of Ukraine next time.

The only ways this could meaningfully be settled would be:

• Ukraine leaves Russia in a materially worse position than before the war, and remains prepared to do it again for ongoing deterrence, so that game theoretically it would be a bad idea for Russia to attack Ukraine again ever.
• Major political upheaval in Russia moves them to legitimately democratic, pluralistic and trustworthy leadership, and Ukraine and Russia then come to a new agreement that both sides will genuinely uphold.
• An agreement includes other countries who will credibly enforce the agreement - e.g. Ukraine agrees to give up some of their territory, and the rest of NATO agrees that Ukraine immediately will become part of NATO and be protected by Article 5 - so that Russia cannot take any more territory from Ukraine ever without unthinkable consequences for Russia.