What is the ruling on an agent benefiting from what the bank returns to him for using his purchase card?

If a person delegates another to purchase an item, and the agent buys it using his bank card, then the reward granted by the card issuer to the agent for using it in the purchase is the right of the agent and not the right of the principal. This is because it is not among the rights of the sale, nor is it provided by the seller; rather, it is from the card issuer to its owner.