Requirements management nightmare...
I'm working on a project in the financial sector as Product Owner. We started with a fairly well designed user journey and mockup and created user stories in the backlog.
Now. While the UI/UX part is most of the time clear, the business logic behind many operations is far from defined, far from stable, constantly changing and topic of lenghty discussions among business experts.
As we are dealing also with a fair amount of legacy, technical discussions must also be involved in business discussions.
The final logic must also be "accepted" or signed-off by some manager.
Result: now the "requirements" are scattered among several stories, I've serious difficulties in following conversations, understanding what has been implemented, how, why, and approved by who.
We really need a "process" to manage this mess. Just checking if somebody was in a similar situation and looking for suggestions on how to get out of it...
In the world of incremental delivery, requirements evolve as an increment is delivered. Each time a Sprint Review is done with the stakeholders, the next set of requirements are finalized. That doesn't mean that you shouldn't have a long term vision or goal for the product but the details emerge as work progresses. This practice is to help address the exact situation that you are encountering.
Scrum offers you a framework to help "manage this mess". The process is something your organization will have to create. I suggest that whatever process is created, it uses empiricism as a basis. Acknowledge, upfront, that you can only make decisions based on what you know at the time the decision is made. Also acknowledge that as soon as work begins on something, new knowledge will be acquired and adjustments will need to be made. Embrace the changes rather than trying to fight against them.
The final logic must also be "accepted" or signed-off by some manager.
Considering everything you've said, are they really in a position to accept or sign-off anything? What do they know that you don't know?
The Product Owner, and not "some manager", is accountable for optimizing value, Sprint by Sprint. Work with the Developers -- who are the experts in the field -- so the team learns to build the right thing at the right time. Each Sprint is a learning experiment and a project in its own right, and each ought to be conducted in the spirit that it might potentially be the last.