Settelin amount increace daily

I bought a loung suite by OK furnishers in Brits. My last instalment was due on the 1 July 2015. My instalment every month was R996.81 I paid R1 000 every month from 1 January 2013. Last Friday I went to settle my account they told me the amount is R1 071 that is more than my instalment we ask them for a statement then load shedding started. We did not get a statement and they told me the extra money is for intrest. We went back yesterday to get a statement and the amount went up to R1 127 again. We spoke to the manager Albert Louw he said it was intrest. I did not pay because they told me somebody from rainbow finance will call me tomorow with the correct amount. They did phone me today 2 July and the amount went up again to R1 138. Everytime we ask for a setteling amount it increases. What happend to the R5 I paid extra every month and how is is possible if your last payment is due you have to pay more than your monthly instalment. I am very unhappy about the service we received and all the interuption that took place while we were in the managers office. They did not even attemped to get a answer all they said the whole time was the money is for interest.