Final answer:
The sale will be upheld under the legal principle of agency law, which grants authority to agents to act on behalf of the principal.
Step-by-step explanation:
The legal principle that will uphold the sale in this situation is the agency law. According to agency law, an agent has the authority to act on behalf of the principal and bind the principal to contracts.
In this case, the real estate agent was hired by the property owner to sell one property, but received a strong bid on another property. Even though the owner claims that he never gave the agent authority to sell, the agent's actions can be considered within the scope of his agency and therefore the sale can be upheld.
It's important for property owners to clearly define the authority and limitations of their agents through written agreements to avoid any misunderstandings or disputes.