BILLING NOT IN LINE WITH CONTRACT

As per my contract (which was reduced), my monthly instalment should have been aligned with my revised contract. My Dec deduction was R1540 whilst Jan (which was when my revised contract was effective) R2258 was debited from my account. How is this possible? How can you just debit any amount from my account? If the difference is not in my account by Monday, 5 Feb, I will be reversing the entire debit order and cancelling ALL my contracts with Old Mutual. This is *****!!