Some offices may preserve an amount of Juvederm or Restylane that you truly did not need on that day, so that you may use it later. Other offices throw it away and tell you they can't save it. Still other offices may put filler into you where you really do not need it, in order to say they aren't saving or throwing any away. Be careful to pay attention to what is really going on, so that at least you understand the situation.
The prices charged for these treatments are set by each office, and can vary depending on what you are actually being charged for. In some offices, there is a fee for the physician's injection built into the initial purchase of the material. In these offices, it may be standard for them to charge a small injection fee on a second treatment day, since you have already paid for the material, but are taking some of the doctor's time on a different day, for an additional procedure. Some offices charge more for the material to begin with, and roll this type of addtional treatment into their upfront costs. It isn't really a matter of "fairness". It's a matter of knowing what that office feels they are including in their fees. It is good to know and ask about this type of thing.
The unfair part would be if they are truly pulling a bait-and-switch, not having fully informed you of potential costs before you started treatment.