The credit still shows in my account. I'll keep an eye on it.
I just checked, and realize perhaps you don't have enough Xbox money to cover the full purchase... which would change the answer. (I have $180 in my account so never thought to check into this)
"If you used account credit only to pre-order your game, your balance will be taken immediately. If you used a credit card, or split your payment between credit card and account balance, you will be charged around 10 days before the game launches."