The right of a jewelry seller to deduct 25% in case of returning it

I purchased a diamond Jewellery 15-20 days back from a renowned jeweller in the UAE for a price of AED 20500 and made advance payment for AED 15000.

Due to some unfortunate reasons, I had to cancel my purchase. The jeweller is deducting 25% of the paid amount ( 5% for vat and 20 % being there policy) to return the cashback.

I believe the amount deducted is unreasonably high as I have not bought the jewellery back home since the amount is not paid fully.

I offered them an option to allow me to purchase gold jewellery instead and they still are very stringent in deducting 25% of the amount paid. It has only been 15 days since I made the purchase, I don’t know what to do.