I was recently promoted. There was no hard number attached to the promotion immediately, so when I met with my manager & the VP to discuss my new responsibilities I took the opportunity to put a number out there. A week later when the number was actually delivered to me it was more than 15% higher than the one I had suggested. My manager explained that they didn't want to reduce my salary just because I had misread the situation.
So, yes; within certain tolerances they will pay you only what they have to pay you to keep you as long as they need you. What you have to understand is that while this is just business it isn't always inhuman.
You have to think of yourself at all times as a single employee business who's interested in maintaining a relationship with a larger business to mutual benefit. When it ceases to be mutually beneficial then someone will end or change the relationship.