I'm Trying to Make Everyone Happy

That's a quote from Toronto's mayor David Miller in the Toronto Star.
Disclaimer (I think): This is NOT about politics.
It's about reality.
A search of the web for "all of the people all of the time" turns up more stuff than I care to read, but the quote "You can fool all the people some of the time, and some of the people all the time, but you cannot fool all the people all the time" is attributed to Abraham Lincoln, and he's not telling where he first heard it.
Someone else, more recently, could have been Gerald M. Weinberg in one of his books talked about a triangle, the apexes being Dollar Cost, Time, and Features of a project.
Both sides need to (not "ought" or "should", but Need To) agree that a project will contain such-and-such features, written down, quantified, spelled out to the best of all parties abilities and with the utmost sincerity.
Both sides need to (not "ought" or "should", but Need To) agree that a project will be completed for so-many dollars, with the contributing sums identified against specific features, written down, quantified, spelled out to the best of all parties abilities and with the utmost sincerity.
Both sides need to (not "ought" or "should", but Need To) agree that a project will be completed by such-and-such a date, with milestones, or objective dates, written down, quantified, spelled out to the best of all parties abilities and with the utmost sincerity.
An overview team meets once a month, or once a billing cycle to compare target dollars and target dates against those in the specifications. Any discrepancy raises red flags and can prompt a project meeting.
Chances are strong that we really did not recognize a problem (feature/dollar/day) when we sat down in good faith. If it really was good faith, we should correct our mutual misconceptions now, not tomorrow, not when the final bill comes in and swords are being drawn.
You can change the apexes on the triangle, one, two, or all three of them, but you can't do that without changing the area of the triangle, and hence the impact of the project.
Anyone who thinks a change in features, dollars or days doesn't really change the project is one of those being fooled this time around. And chances are strong that they are fooling themselves.