The "Negotiate" of I.N.V.E.S.T. - How much and when?

Scrum practitioners:

What is your experience with the degree and the right time to "negotiate" on a story?  Is this done purely during PBR, and when a Sprint begins, the story becomes"fixed?"  (Even though it's not supposed to be "a contract?")  Or during development, as collaboration occurs, does "negotiation" also occur?  I'm sure there are risks to being too strict either way....

If you allow "too much" negotiation during the sprint and are always negotiating about what can or can't be accomplished, it could be that:

• things (stories) just never get done because the goals are constantly being re-negotiated 
• metrics are all screwed up and capacity and velocity are meaningless
• therefore, estimating becomes meaningless
• no one feels accountable to accomplish goals
• no one is held accountable to accomplish goals
• the team is engaging in an anti-pattern and isn't holding themselves / each other accountable
• project goals aren't reached in a reasonable time
• and ultimately value isn't delivered. 

If you force all negotiation to occur in PBR before a story is accepted into a Sprint, then it could be that:

• collaboration is forsaken in order to get this agreed-upon story done (just git 'er done!)
• the sprint builds something that is wasted because the opportunity to refine the goal was missed
• findings are not brought back to the PO to inform refinement of other stories

It seems to me that a rule of thumb would be to fix the stories in advance of the sprint (negotiate ahead of time, during PBR), accept the risk of "waste" during the sprint (a story may be produced that is ultimately discarded b/c, as we later learn, it did not provide value), but also force (foster) collaboration during the sprint to bring the learnings into future PBR.

Has anyone else encountered this balancing act?  Either too much negotiation during the sprint and always moving the goal posts?  Or too little allowance for negotiation during the sprint and creating too much churn and too much waste?   Has anyone else fell off this tightrope and have the bruising and scars to show for it?

What are your lessons learned for when and how much to negotiate?  

Thanks in advance.  And if I missed them, please point me to other forum entries where this topic may have been discussed before!