bad service from Huurkor

I stayed at flat, managed by Huurkor, for 5 years & moved out last year. Before moving out an exit inspection was done & I was told the place looks ok. I was surprised to then hear from Huurkor that the walls in the one bedroom were dirty & needed to be painted. After contesting this we came to an agreement that the walls will be washed, that money will be deducted from my deposit & the difference will be paid into my account. Indeed an amount of R4200 was paid into my account. Almost 3 months since I moved out, I got an email from the same Huurkor agent telling me to deposit the R900 for the cleaning of the walls into the huurkor's account within 7 days & failure to do that will result in Huurkor debiting my account. I rep**** to this threat that as per communication between me & them, they were supposed to deduct that R900 from my deposit & refund me the difference. This person, without explaining continued to say if I don't pay they will either debit my account or blacklist me. Mind you I never received proof of how much my deposit refund was (before the deduction and after the deduction). I do not appreciate their unprofessionalism and I am actually fed-up with their attitude.