Creditor negotiating in Bad faith

I had a long standing debt with Woolworths I have been struggling to get on top of with not much success where I was paying R500 for the past couple of years. I was then offered a settlement discount to pay the debt off over two instalments of R5813.00 which I paid end of November, and the October R500 reduced the final december instalment to R4813 one of the ladies that work for the collection lawyer company had said. I then paid the balance of R4813.00 belatedly on the 2nd of January with the expectation that the debt was finally settled, only to get a call that I still owe the company over four thousand rand which equates to the discount they had given me. I just find the argument that I had paid late unconvincing as the payment was made and they should stick by what they had promised me and not come with convenient excuses of the date they alleged they had told me to pay. I just feel they had negotiated in bad faith in duping me to the onerous commitment of the exhorbitant amount I had with pains had to stump for the R5813.00 and the R4813.00. I feel it is not good business practice to get away with a case like these to fleece people like this.