1) You can use a debit card to take out a car, only when showing a return flight ticket (not just a reservation).
2) Without the return ticket, you must use a credit card when you take out the car.
3) When you return the car, whether taken out via credit or debit card, you can pay with a debit card.
ALAMO DEBIT CARD POLICY (from website):
. . . Debit/check cards are considered to be any non-credit card bearing the VISA, MasterCard or Discover Card logo.
. . . Any other non-credit card without one of these logos is not accepted.
. . . For pick-ups in the United States, without proof of round-trip ticket, debit or check cards are accepted only when returning the vehicle.
. . . A credit card in the name of the renter must be presented at the time of pick-up.