FNB LOANS

I had a loan with FNB that I settled in full on 2019/07/18. I reveived my settlement letter on 2019/07/19 from them. On 2019/7/31 again they took money out of my account. On the same day I phoned them and they promised me that it will take 5 to 8 working days for them to refund me. On the 2019/08/08 I phoned them again to enquire how far and they informed me that they did refund me the money BUT not even a 8th of the money was refunded. Then they asked me again for the settlement letter and I mailed it again to them and my proof of payment again and once again they promise 5 to 8 working days. Then after hearing NOTHING from FNB I phoned this morning again. Now they tell me that they do not own me anything because the settlement amount that they gave me on 2019/07/18 was incorrect and that is why they debited money out of my account. All the years I praised them and complimented them from 2010 when I opened my account. But I am done with them. They really unprofessional in the manner they treated me and I will move all my stuff to another bank. As far as I know once you give me a settlement letter and I settle the account IN FULL AND GET MY SETTLEMENT LETTER the agreement of the loan is done. NOBODY GAVE YOU PERMISSION TO TAKE MONEY OUT OF MY ACCOUNT AFTER OUR AGREEMENT FOR THE LOAN WAS SETTLED FNB.