Most suppliers on Northbound require a credit card to cover the excess amount.
Some offer the possibility to pay it with a debit card or cash but you'll need to buy extended insurance with your booking like SCDW ad more.
Once you return the vehicle you'll get that money returned if there's no issues with the vehicle.
If you have a credit card, most suppliers only take an imprint of it when you arrive and don't actually withhold any amount from the card.
If you select a vehicle you can click the Important Information tab to see more info about that suppliers policies.