The right to dismiss a wakeel remains with you all the time, e.g. you say to a person: “I need a goat. If you come across one, you must buy it for me.” Thereafter you stop him from purchasing it for you. He now has no right to purchase it for you. If he purchases it, it will be his responsibility. You do not have to take it.
You did not dismiss a wakeel yourself. Instead, you wrote him a letter or sent someone to inform him that he should not purchase the item for you. Even then, he will be dismissed. If you did not dismiss him yourself, but someone else went on his own and informed him that you have dismissed him and that he should not purchase the item, then he will be considered to be dismissed if two persons informed him of this or one reliable, religious person informed him of this. But if this was not so, he will not be dismissed. If he purchases the item, you will have to take it.