ICEAP did not refund deposit due to visa not refused

The class was supposed to start on August 29, and I paid a $7,800 deposit for the four-month language course. However, I informed the language agency on August 19 that I was going to change schools and would not attend classes and asked for a refund of the deposit. The agency staff told me that it would be refunded at the end of September, but now they told me that it could not be refunded, because the offer stated that "the deposit can only be refunded if your visa is refused, and other cases will not be processed". I think this clause is very unreasonable, I have no demand to take classes, and I have been notified ten days in advance to withdraw my seat. I didn’t enjoy the course service, and I had to pay so much deposit. I informed the institution before the course started. They delayed the refund until September and told me that I couldn’t get any refund.