Why Is my xbox forcing me to pay a specific game by card?

Do basically, I'm trying to buy the new Call of duty: Modern warfare 3 and my xbox keeps asking me for a Mastercard approval yet I'm trying to buy it with Microsoft account money. I have enough for the game aswell, so why is it asking me for a Mastercard approval? It's not cause its PG18, because I've bought a different cheaper game for £4.00 and it let me without asking. Can someone help?

:)