FNB took my money wrongly

So I sold an iphone 11 I had bought from someone who was immigrating to another person a friend for R18700, I specially asked them to make my reference iphone sales which they did. On Friday the 11th FNB dedcuted about R5000 from my account so I decided to go to their office in sunninghill to find out what’s wrong and they told me the R5000 has be help for ***** reasons which really ****ed me off because I sold my phone I had bought with my hard earned cash and I don’t see why I’ll be labelled as *****. They try to reach The investigation officer and he’s rude reply was that I must go get a proof that the phone is mine. How would I get a proof of that nature when I didn’t buy it from the store or even use it for a day. If you buy and sell items from Facebook market you’ll understand that you don’t need proof or need to investigate where the buyer’s money comes from as far as the money is in your account you go.FNB threatened to blacklist me for life for a ***** I know nothing about. They must give me back my money.