Incorrect Debit Order

I purchased a Dell laptop and tablet in November 2014 and since then FNB has debited my account incorrectly. The laptop was advertised in both the FNB brochure and FNB website as R299 p/m but FNB have been debiting my account for R315 p/m. The tablet is correctly debited for R265 p/m though. I went into the FNB store in February 2015 and Candice acknowledged that the incorrect amount for the laptop has been continually debited from my account. Candice tried resolving the problem by speaking to a colleague of hers telephonically in my presence and I was told that everything should be resolved within 7 days but to date its still gone unresolved and FNB continue to debit my account for R315 p/m instead of R299 p/m.