They would want a legally binding guarantee that Ukraine will not join NATO before they would hand Crimea back.
Also they would demand that the people of Crimea decide whether they want to be part of Ukraine again.
Putin has illegally annexed Crimea, illegally invaded Ukraine and illegally murdered dissidents with radioactive poison (again illegal), suspicious windows and and suspiciously exploding airplanes.
Just how legally binding could any agreement with Putin and Ukraine be? You're willful naivety is beyond contempt.