It sounds to me like you're starting to let the "process" take over the
process. IMHO, as long as everyone understands what's going on, what needs
to be done in order to go forward, and the team understands the relative
priorities then you're on the right track. Splitting hairs over how and
when a task gets re-evaluated, re-estimated and rescheduled is a waste of
energy. The bottom line is that it isn't going to happen this Sprint, it's
way more complicated than anyone thought when it was added to the backlog
and it needs to be addressed later.
If the politics of the situation demand some kind of procedural ballet
dance, then I'd suggest this: Declare the task "impossible as specified"
and mark it as completed. Then you're done. It's the PO's problem to
reintroduce it to the Product Backlog as a new item, with different
specifications that can actually be met.
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