That's what it comes down to. A power of attorney is simply authorization for the agent to act on behalf of the principal. If the agent knows that the principal does not want the agent to do a particular transaction then the agent cannot do it, assuming the principal is competent, of course. It does not not matter that the transaction might objectively be in the principal's best interest. The principal has the right to do things that might not be in his/her best interests.
If the principal is incompetent and the agent holds a durable power of attorney then it's a different story. There the agent needs to act in the principal's best interests and can do the transaction notwithstanding that the principal does not want it. But the agent had better be very certain the principal is legally incompetent before doing it.