Payments without balance going down

I took a loan with wonga of 5000 and unfortunately I couldn’t pay the required monthly instalments . I then organised with them that how about I pay 500 every Friday of the week to pay up my debt with them . They have been deducting the R500 on Friday ever since, then on the 23rd of May they deducted R2 151,98 that landed my balance to R2000 then on Friday the 30th of May they deducted R500 again which brought my balance to R1400 and again they deducted R500 on Friday the 6th of June but my balance is still standing on 1400 till to date . What they keep doing is add on interest even when I paid them because now the balance is R1 420,66 . They say bank payments take up to 2 to 3 days to reflect and it’s about close of business today and there’s no changes to my balance . Whilst tomorrow midnight they will be deducting another R500 from my account. Because according to my calculations I’m only left with 2 payments then I’m done but they want to **** me my money and not bring down my balance .