Unless they have changed the way this is done, you have to use a credit card to guarantee your reservation (the card will not be charged at the time of booking). This applies only if you already have a dining plan on your room reservation. If you don't, you won't be given that option - you'll have to pay in full when you book it.
You don't have any dining credits until the day you have checked in for your package. They can't deduct credits from your account when you book, because you have no credits to deduct. What happens is that when you pick up the tickets for your dinner show, the credits are deducted from your account at that time. If you don't pick up your tickets, don't cancel and don't show up, if you left a credit card as a guarantee, that card will be charged the full price of the dinner show. You'd be paying for the dinner show you never attended, but you will still have the dining credits because they were never deducted from your account.
You can also switch a prepaid reservation to dining credits payment, and get the prepayment refunded. If you can't do this ahead of time with a credit card guarantee by calling WDW-DINE, you can do it when you pick up the tickets.