Billing me when account was fully paid

I'm a Bcom IT Management student and my company paid the full amount in October 2017. Mancosa came back to me and said I owe them. I explained that my company did make a payment. I later sent them a proof of payment after they sent me a statement, saying that I now owe them R11,280. After following up again with proof that my company did pay, they sent me another statement saying that I owe the R11,280. According to my understanding they owe nothing. I have been reaching out to them via email and phone but they still have not helped me. The student Adviser who gave me the invoice at registration was Lethu who is based in Durban campus. I am very disappointed by the service here!