What are police/FBI negotiators allowed to do?

I was talking about this with my brother just now. if, hypothetically speaking, a group took a large number of valuable hostages, what sorts of limits would negotiators have on what demands they can meet. like are they allowed to alter personal records/legal stuff? and of course there is the question of can they grant you a pardon for taking the hostages in the first place? because after all, what are you gonna do with all your cool new stuff if you just get arrested and sent to prison for a few years directly afterwards? I was just wondering.