Validity of a purchase made on the agent's liability when the principal specified purchasing with the exact coins provided?

Primary text

If the principal specifies that the agent must purchase the item using the exact coins provided, and the agent purchases the item on their own liability (*dhimma*) and then settles the payment with the provided coins, the purchase is not binding upon the principal. This is because when the specific funds are designated, the contract becomes void if those funds perish or are wrongfully possessed (*maghsub*), and the principal is not obligated to bear a debt liability. Since avoiding this consequence was the principal's objective, contradicting it invalidates the transaction, and the purchase becomes legally binding upon the agent.