I've been working as a team lead on a series of tight-deadline projects for a very high-profile client. I make sure going into each one that the scope of work is very clearly defined, with the client's help, to ensure my team doesn't get swamped by unexpected work while they're under pressure already.
Unfortunately my boss, the actual delivery manager for all projects under this client, has a habit of forcefully (and often) changing our priorities and inserting new deliverables that are outside our scope of work, his reason usually being as vague as "We have to have this" or "The client needs this now." After going through this several times I have noticed he is wrong every single time, and is sometimes even completely contradicting something the client stated directly about our timeline or expectations.
What's worse is when this work causes an actual deliverable to be late, he does not hesitate to throw my team under the bus in front of the client, and I can only imagine what sort of image that gives them of our company. Such confrontations have lead some team members to get burnt out and quit the project.
I've tried to diplomatically push back on these requests but the more I push, the more forceful he gets. Is there something I should be doing differently? Or is this manager just a lost cause?
Edit: To clarify, my whole team is client-facing to some extent (I personally deal with the client more often than the manager does, with regards to my projects) and are usually all present when the bus hits. Post-bus, the rest of the day (well into the evening) is usually filled with angry emails/calls/messages from the manager to seemingly random members of the team (rarely me directly, because I'm the only one who pushes back) regarding the status of whatever deliverable was delayed that I would easily classify as "badgering" to get it done.