First check
Compare the linked-account balance and value date with the loan statement for the same days. Confirm how the lender calculates interest, whether every visible rupee offsets principal, and whether any minimum balance, drawing-power or product rule changes the effective offset.
What to do
Rebuild a short period of the interest calculation from the lender’s disclosed method using the actual daily balances. Ask the lender to explain the first date or amount that does not reproduce instead of arguing from the headline overdraft feature.
Done when
Your calculation and the lender statement reconcile, or the lender corrects a documented posting/value-date error and the revised interest appears on the account.
If it still fails
Raise a written grievance with the calculation, dates and statement lines attached. Keep paying the undisputed amount while the calculation issue is reviewed unless the lender gives different written instructions.