Upon whom does the loss of one-third of the land fall: the buyer or the seller? And what are the Shar'i rulings related to this issue?

It is not permissible for an agent to purchase from himself unless explicitly or customarily authorized by his principal. If the agent acts contrary to this and purchases from himself, he is considered an unauthorized transactor. Scholars have differed on the validity of such an action, but if the principal ratifies this transaction, the land becomes his property from the time of purchase, and any subsequent damage falls upon him.