What is the ruling on dealing with someone who does not understand a request for annulment (of a sale) when selling them something and then requesting annulment from them?

If the sale fulfills its conditions and pillars, it becomes valid and binding for both parties. Lack of understanding of iqālah (cancellation) does not affect the validity of the sale. One can request iqālah by explaining its meaning to the other party—that it is the rescission of the sale. The decision to accept or reject rests with them.