Buffer

Whatever a buffer is or contains, the Sprint will end when the Sprint timebox ends. That's it. Any remaining work ought to be reestimated on the Product Backlog. The work may or may not then be planned into a future Sprint.

What is a buffer and how did the Developers provide an estimate for something that isn't actually defined? 

@Ian answers your question.  Sprints end at the end of the timebox that has been set. If a team finds that they have satisfied the Sprint Goal and still has time left in the Sprint timebox, I usually suggest that they spend that time for backlog refinement, technical debt, or learning opportunities for something that would benefit them. 

If I understand buffers in your context, unused time (buffers) should be filled with work that supports the sprint goal and is added to that "time" by in cooperation with the PO.  It's probably stuff from the product backlog.  Keep sprints the same physical time as a rule. 

That said, you CAN change sprint lengths but keep them the same after changing for a long time.  It should be like moving to a new house - disruptive, expensive, and not something you do very often.  Settle into the new place if you do move.  You should also have a really good reason for changing the sprint length - such as "I don't need to look at my progress and inspect and adapt so much become my work is not so risky".