Overpaid and they short refunded

I am an agent. I overpaid and requested a refund. They short refunded by over R3000 stating I had unpaid invoices. Obviously.... a load of nonsense. I called... very unhelpful and rude. I was asked why I overpaid. Line manager just as useless. Very disappointed at the service they given to their agent. I can only imagine how they treat their customers. Well my account shows I'm in credit by over R4000. WoW