My parents recently were granted power of attorney over my aunt's affairs; she also lives in another state and has dementia. Sadly, though I cannot give you numbers, I know that this was not a cheap or easy thing to do. They had to hire a lawyer, go before a judge, deal with family members who were contesting the decision (my aunt was too incapacitated to object, but I imagine that if she had, that would have added extra time and complication.)
All that to say, I don't think that power of attorney is the way to quickly get out of this land sale. (and I'm also not sure there's any 3 day cooling off period.) Is there any way you can get a copy of the sale documents?
I think what you may have to end up doing is just letting this sale go through for now, pursuing POA, and then at that point, you try to re-sell the property.
I'm sorry you are having to go through this. From witnessing what my parents have had to through, I know this is a tough, complicated road.