Answer:
Undisclosed principal
Step-by-step explanation:
Am undisclosed principal in an agency relationship is one whose existence is not known to the third party. The third party believes they are making the transaction with the only agent involved in the transaction.
In this instance Sarah believed she was selling to Alice and was not aware Alice has a principal (Harold). In her mind she sold the land to Alice and no other person.
It was at the point where Harold said he no longer wanted the land that Alice told Sarah about him. At this point the contract between Harold and Alice had been terminated