The plastic surgery clinic gives a discount but does not provide a cash receipt.

I'm going to get plastic surgery at Apgujeong ㅇㄷ Plastic Surgery Clinic next Monday. I heard that if you pay with a card, it's 1 million won, and if you pay in cash, it's 800,000 won? So, I deposited 800,000 won by bank transfer, and all I had to do was go for the surgery on Monday. Then, after her surgery, my mom told me to bring the cash receipt and her year-end tax receipt. So, I thought they might not be able to do it because it was a plastic surgery clinic, so I told my mom to call me directly. When I called, they said they wouldn't give me a receipt even if I paid in cash because they gave me a discount -_- No, even if they gave me a discount of 200,000 won, I still paid in cash, so I still have to give them a cash receipt, right? I heard that if you report a cash receipt refusal, you can receive a reward of 20% of the payment amount. Should I just say that I knew they wouldn't give you a cash receipt and report it? Hahaha.