Bunch off reapoffs

Had a nedbank credit card with nedbank with a credit limit of R7500 which I maxed out. Lost my job and could not pay it. It was handed over to this tsotsies whom I contacted to make an arrangement with. The outstanding amount was about R9000 with interest and staff which was handed over. The amount changed from R9000 to R15325 ( it has remained at this amount for months before I had made the arrangements) which I accepted to pay. Made an arrangement of paying R3000 a month of which I already payed R10000. This week I sent them an email requesting the outstanding amount and I'm being told that the outstanding amount is R7723.18 of which is plus 2700 more. I queried the amount and I was told it's the Interests accrued from the amount which is according to the duplum rule. According to my knowledge the rule Literally translated, in duplum means ‘double the amount’. A part of South African law since the early 1800’s, it is a common law rule that specifies that interest on a debt will cease to run when the total amount of arrear interest has accrued to an amount equal to the outstanding principal debt. So R7500 x 2 is equal to R15000. Where do other amounts come from? They are a bunch of ********* preying off the poor pple who don't know the law