Amended their bill after I had paid them in full and then handed my account over to a debt collector

I do not have medical insurance and always insist on paying for medical visits before I leave. Imagine my surprise when a sly debt collector (Schulenburg Attorneys) contacted me to recover R135 plus about R250 in fees. After I provided proof that I had already paid for my visit to Dr van Onselen's rooms, I was informed that the amount was amended after my visit. I never received an updated statement or even a phone call from his people. I offered to pay the R135 plus interest but I refused to pay the additional recovery fees. After more than 4 years of harassing emails and calls to both me and my wife, they eventually backed off. Since they never accepted my offer, my wife and I decided to donate R500 to someone in need in stead.