You cannot be agile if your client insists that you work in a waterfall. If they demand estimates (which they see as commitments) and then set a time/money budget based on those estimates, that's waterfall. That works against them, of course. If the time/budget is fixed, then scope must change. If they won't permit that, you are being set up for failure and are wasting you client's money to boot. Are you willing to eat the cost overruns or the costs of the changes that will be required as the project develops (or deal with a very unhappy client if you're unwilling to do that)? IMO, we all have an ethical obligation to to explain that up front, before accepting a job.

Source: x.com/allenholub/status/1820918589370867836