***** at old mutual to their clients

I took out a loan with old mutual, so this month was my last installment, I settled the installment just before pay day, as I was using their app, it could not load 0.76c, so I left it out. Come pay day, they deduct the monthly installment as per normal, I called and a consultant said I will be fully refunded. Days later I received my money which was R1000 less, went to the brunch and I was told the 0.76c has accumulated the normal interest. How on earth can 76c accumulate 1000 as interest, did I not pay the interest when I payed the settlement. I was double charged, **********ly so.