What is incumbent upon me after learning that the buyer defrauded his cousin, and am I obligated to compensate his cousin or inform him of what happened?

The questioner must return the amount exceeding the agreed-upon price (12,000) to the principal because he directly took the excess, in accordance with the Prophet's (PBUH) saying: "The hand is responsible for what it takes until it returns it." The questioner then has the choice to claim the amount paid to the agent or to forgive him.