standard bank charging customer for their own mistakes

ok... the lady from standard bank that I spoke to doesn't understand accounting priciples.. so standard bank messed up one of my accounts by closing by transfereing money into it by error... they fixed it which i am still grateful for... but now because they reopened it and reversed the money, they did not reinstate the debit order, so i defaulted on my September payment - .... come October i asked for a credit increase. so when they worked out the new instalment, they take in account the outstanding balance and the new loan amount to work out a new instalment... so the interest on the previous amount was calculated and added to the outstanding amount..so how can they now demand i pay that September instalment when the interest was already included in the new outstanding amount... especially when it was their mistake... not mine.. its like they punish their customers for mistakes that standard bank made...