Hong Zhu car rental now MHX group rental

[Stunt_101]

I recently rented a car from this car rental company call MHX group rental (formerly known as Hong Zhu) on Midland avenue and Finch Ave East.
they told me a $500 deposit was required upon the rental and will be refunded after i return the car.
so i used my debit card for transaction so it can be electronically recorded in case anything goes wrong.
Upon returning the car, they told me that the money will be available back into your account within 5-7 working days.

My question is: How can they put money back into my account without me authorizing the transaction? Are there any possible way that they can deposit the money back into my account without my authorization? It was listed as a Point of Sale purchase in my bank account.

Thanks

Edit, I called my bank and finally got through and the CS rep told me that it is possible for them to issue a refund without my authorization and the money will be automatically transfered back into my account.