Such a request would be rejected because we don't add or remove the items in the Sprint Backlog
(while we always add and remove tasks). However, this is up to the Product Owner to reject this
request, rather than the developers. So, they should just refer the customer to the Product Owner.
In very special cases, the new feature might be so special and the project environment so influenced
that the introduction of the new feature makes the current Sprint Goal invalid and therefore the
Product Owner will cancel the Sprint. Remember, this doesn't happen often.