Charged 2k+ for using dollars

I went to buy a phone on Sunday at Vodacom 4U in Sandton City, but the problem was that I was using US dollars and I asked if they accept dollars and they said yes. The phone I bought (Samsung A35) was R8 500, I gave them $600 without even calculating and the total payable for me was R8 748 but I was given R255 change. I asked the girl assisting me not to open the device because I wasn't sure of model, but she insisted on opening even I said I'll take responsibility if it's physically damaged. When I got home and did the calculations, I was amazed to find out I charged R2252 just for using a foreign currency and I was never even warned about that before completing the transaction. The following day I went back to the store and they gave me a run around, I was supposed to wait for the manager Mishack, I was only able to find him when I went back the second time but he didn't help me. I asked for a refund but I was told I couldn't because the phone was opened...and who insisted on opening it? I'm infuriating as it is...this was the worst service I've ever received in any mobile network store I've ever been to. I've sent an email to Vodacom customer care and I only received a ***, but the agent I spoke to when doing a follow-up says it doesn't register on their side.