It will work the same as a card payment so if the credit is more than the deposit, the system just takes the amount for the deposit now and then will claim the rest from the credit when the balance is due 4 weeks before the booking begins. If the booking is more expensive than the credit you have, the remainder can be paid by card.
If for any reasons the credit does not deduct, please let us know and we can apply this manually for you. Please do let me know if you have any other questions and we would be happy to help.