I just used my credit on Saints Row IV, which worked, but I have no plans to buy Ultra SF IV.
Can I split payment between my local currency and a credit card?
Yes. For most purchases, if the amount of local currency in your Microsoft account is not sufficient to pay for the content, you can charge the remainder to your credit card or PayPal account. PayPal availability varies by region. However, you must first select to purchase using your credit card or PayPal account and then you will see the option to use the remaining currency in your account. Split payment may not be an option for all content purchases.
right from their site. clearly they dont tell you which ones allow you or dont allow you to split the purchase. Which is again, playing roulette with your money. I clicked purchase on SFIV hoping to god a message would come up, instead my card is charged the full price, and Im sitting with 15 in my account
If you're buying from the Xbox dashboard and you have a credit card on your Xbox account you can use it as a payment method and then if you had money on your account it will ask you if you would also like to apply that money to what you're purchasing and pay the rest with your card.