What paperwork have you had to get her to sign?
Was the agreement concerning payment of £3,000 in writing or email, or was it merely discussed orally?
It sounds to me that if a power of attorney had been set up, she would have agreed to sign this paperwork when she first "sold the property" and hence, she was only doing what she probably legally agreed to do in the first thing.
If you had agreed to pay her the 3K by email, in theory, she could pursue you for this, so you really need to find out from the Company who purchased the property from her as to whether the small print at the time made it a condition that she would sign any future paperwork in the future. If it did, she doesn't have a leg to stand on.
In any event, if I were you, I wouldn't be paying her a penny! She would have to take you to Court and even if she did, she would need your forwarding address before issuing any Court proceedings can be issued.
I hope this helps as far as it can.
It would be usual for her to have been under a duty to sign, but without seeing the paperwork myself, I can't of course give you this confirmation 100%.
In any event, if she has previously sold the property, she is not legally entitled to any further monies relating to the property.
I would just ignore her from now on!
If I have assisted, I would be grateful if you could rate my answer.