SSD NOT HONORING AGREEMENT

I have made an arrangement with Thabang Pooe from SSD to pay an account in full. I have asked her if I settle the amount in full will I get any discount. She confirmed in writing that I will get 30% discount but first I had to pay a certain amount before she could calculate the discount. This I did and she did send me confirmation of the amount still due after the discount of 30%. I made an arrangement to pay this amount on 22 February 2018. The arrangement was made on 5 February. When I made the payment on 22 Feb as arranged she sent me an email stating that the discount was calculated incorrectly and they cannot close my account. This is not acceptable to me as she had 3weeks to confirm this to me. According to me I have settled the account as per agreement - they cannot tell me on the day I have made the payment that the amount is incorrect.