DAY LIGHT ******* TWO AMOUNTS IN ONE ACCOUNT IN ONE MONTH

On the 11th July 2019 I received an email of the amount owed for my handed over Woolworths account from Debt-In giving me a figure of R6 828.25 owed. I get an assistance to settle this account um struggling with for years to pay up. With the little airtime on Friday the 26th July 2019 I call the Debt- In Collections that I will be settling account and I received email saying I owe R6 828.25 Mahle the consultant refused the amount to say I owe R6 906.40 I asked her how did the amount get to that in a same month and said it is interest received from Woolworths their client. I was shocked for that interest in a month be given to a person struggling to pay the account hence it is handed over???? Like really??? So on Saturday my assistance pays R6 900 to Debt-In and I called Monday to request a paid up letter and it was said it will be emailed to me to date I have not received the Paid Up Letter so I call the Agency spoke to Lindokuhle who transfered my call to Mahle who says I'm still owing R6.40 therefore she won't issue a Paid Up Letter. I got assistant to pay the account off now R6.40 of which is the interest added to the account in a month is blocking me to receive the Letter. This is truly a day light *******. Woolworths please answer me.YOU SEE WE STRUGGLE TO PAY BUT YET YOUR INTEREST ADDED TO OUR ACCOUNTS IS SHOCKING. MAY I PLEASE GET A PAID UP LETTER FOR THE ACOUNT THAT WAS PAID ON SATURDAY 27TH JULY 2019. SO I HAVE TO AND ASK FOR R6.40 AGAIN?????