Credit card company greed

An "oustanding authorisation" (in connection with a deposit of 1500 Euros on a hired car) placed on my credit card was terminated by the hirer on 12th September (Monday last week, when I returned the car) but has still not disappeared from my account. An enquiry to the bank produced the response that the card company sits on the amount for TEN working days after cancellation, earning interest for themselves during the period and reducing the balance available on the card. I consider this to be usury. The bank maintains the matter is out of their hands, which is an unacceptable response; they should either have negotiated better terms with the card company (Mastercard) or be reviewing the continued relationship with the card company.