I thought bait and switch techniques only happen during a sales process. And to defend against that is easy because you expect it. You can see it coming.
But what happens when it happens to doing favours? For example, client whoasks for a favour to reduce a cost on such and such a service or items you provided. When you agree, they proceed to ask for the same reduction on other items at hand.
Hey, I promised you an apple. Not the orchard.