Misleading clients

Good Day , I bought a 100GB for R329 only to receive 90GB for the same amount. I lodged a complaint with MTN regarding their false advertising and only for them to respond to me today that I have been allocated 90GB and there is nothing they can do. Even after I have attached proof of what I bought and also showed them the billing price is still the same. My number is *** and ID no is ***. Either they cancel this contract or they fix the error and give me what is due.. This is ******* and misleading customers