Wessels & Smith worst nightmare

I have been handed over to Wessels & Smith attorneys for collection. My debt was R2800 of which I managed to pay R5100 including charges. Mind you during the period it was a struggle to get statement/balance after each payment. In December they sent a summon of R2800 meaning back to the initial amount. I payed R3500, then in May when I asked for a statement they said I still owe R2000. Are those charges/ interest legal in South Africa?