Bonitas collects their mistake monies!

I have been handed over for debt collection. I cancelled a dependent in 2016 and bonitas failed to notify me that they have over paid medical expenses for that dependent at the time of cancellation. The medical fund was paid by the company. I was only notified in december 2018 after changing to another medical aid. The medical aid failed to collect the outstanding amount in 2016. They never notified me of the outstanding amount due to the medical aid. The section on the statements where "monies owed to medical fund" was left at R0.00 all this time. I contacted the medical fund to find out what is going on as the only info i recieved was the call from the debt collector.i escalated the matter,i sat on a call for almost 2hours and they still cannot explain why the failed to notify me of the outstanding amount when cancelling,and why it was left for 2 years with no mention of it. I am still waiting for them to get back to me.