I was told that tax exemption procedures had already been completed, so they could not take it back and give me a refund.

In Japan, whether the purchased item is tax-free or not, as a rule, once the purchase is made, the item cannot be returned. However, some stores may accept a return depending on the reason, so please check with the store. Other things to check would be the existence of written terms, such as the period of time within which returns are accepted, and what goods are not allowed to be returned. If the item was defective from the time of purchase or the staff gave you a false explanation that you believed that convinced you to buy it, you will be able to return the item and get your money back even if the tax exemption procedures have been completed.