DMC - Capfin are trying to Rob me

I have been paying my capfin account through DMC as it's been sold to them due to non payment, my installment was R620 and I have been paying R650 and it reflected on my credit score that I pay R650. Now due to financial problems I made an arrangement to pay R470 per month but last month I paid R650 and they put it on my credit score as I paid R470. I called them and they said i must send am email which they did not respond to or attempt to call. Now they expect another installment but they ****** me of R270 last month. How do I get through this?