Trying to identify a short story (pre 1985) about a human soldier who finds himself locked in a cell with the first ever captured alien enemy. Turns out the alien is a sacrificial peace envoy. In order for a peace treaty to take effect between the two civilizations they must agree to kill themselves as symbolic sacrifice. At fist the human soldier is horrified and rejects the notion. They debate why one is willing to risk their life for another but not sacrifice their life to save many. In the end they come to agreement and detonate the explosive that has been put in their cell for that purpose.
It definitely sounds like C.J. Cherryh, "The Scapegoat", only it was published in 1985 and it's a novella, not a short story. Plot summary from Wikipedia:
An unarmed human starship and its crew of fifteen hundred is destroyed by a technologically less advanced alien race, later called elves by the humans because of their resemblance to the mythical creatures. Other unprovoked attacks follow. All attempts to negotiate fail; the elves fire without communicating. Eventually, the overmatched enemy is driven back to his homeworld, but the conflict does not cease. The Alliance, one of the three human power blocs, ends up mired in a twenty-year-long war.
In all that time, humans get no closer to understanding why the elves fight or how to make peace. A few districts remain puzzlingly neutral, but when humans try to establish relations with one of them, it instantly joins the enemy side. In addition, not a single elf is captured alive; except for the very young, they have the ability to stop their hearts at will, and use it.
Then one day, Second Lieutenant John deFranco takes a prisoner, one who deliberately allows himself to be captured. The creature speaks English, learned from a human prisoner, and calls himself the saitas. He tells deFranco he has come to try to end the war. The elf is passed along to Alliance scientists, but cannot communicate with them. He asks to speak to deFranco, a fellow soldier.
As they talk, deFranco learns that the elves do not comprehend the concept of a treaty, written down on a piece of paper, and do not trust it; their way of thinking is too alien. The saitas explains that he has come to be killed, so that his death will carry away the mistakes of the war. He is willing to sign the human treaty, but the elves require a human saitas. The elf hopes that deFranco will be that one. Meanwhile, all along the front, the elves attack with the little they have left. One of deFranco's friends becomes a casualty.
When deFranco realizes what the prisoner wants, he tries to leave, but the meeting place has been locked from the outside by his commanders and a grenade pointedly left inside. In the end, deFranco finds it within himself to join the saitas in completing his mission. When the recording of their deaths is broadcast, the fighting ends. An elvish delegation arrives and takes away deFranco's body for burial. The humans in turn take the elf's body to be interred on Downbelow, the Alliance world.