Still being billed once account has been settled, now being threatened

My grandfather's employee, Ms V. Ncube, opened a loan account with Joshua Doore, with payments commencing on 01/12/2012. Her initial repayments were R782, but from April 2013 she paid more most months in order to pay off the loan sooner. In September 2014 she made what should have been her final payment, and in fact overpaid. The balance remaining as of 28/07/2014 was R267.69, and on 04/09/2014 she paid R900. Her account then showed a balance of (R633.69), which should presumable show credit owed to her. She did not realise at this stage that she had by now overpaid her account, and made a further payment on 05/10/2014, bringing her credit balance up to (R1563.69). Despite being owed credit by Joshua Doore, she was harassed by Joshua Doore staff who persisted in phoning her to demand payment. My grandfather wrote to Joshua Doore on 1/10/2015 when Ms V. Ncube had failed to resolve the issue in store. In his letter, he stated the following: 1. Ms V. Ncube is entitled to a refund of R1563.69 plus interest (at the same rate that she was charged) for the period September 2014 to the date of your refund to her. 2. The monthly insurance charge of R148.95 fell away after the 20 month period it took to settle the debt. She is, therefore, entitled to a refund of R595.80. 3. Also, the monthly service charge of R37.40 fell away and so a further R148.80, plus interest, is therefore due to her. This letter failed to resolve the issue. The issue was, in fact, compounded by the fact that Ms V. Ncube was then informed in February 2016 that the account had been written off. How can you write off money which you owe to another person? My Grandfather then sent a further registered letter, collected 13/04/2016, to which he has never received a reply. Ms V. Ncube continues to receive phone calls demanding that she owes an amount of R2000 still (despite being told that the account has been written off, and despite the fact that it is she herself who is owed money by Joshua Doore), and threatening that this will increase to R3000 if she fails to pay. You are preying on the most vulnerable, and on the good customers who settle their debt promptly. This is unacceptable conduct, and we trust that the matter will be resolved timeously. All documentation is available to verify the above information.