Choosing How to Pay for Items in iTunes
For an early Christmas present, I just got a new iTunes gift card. (I love those things!) One thing I've been wondering for some time now is after redeeming it, which I've already done, is there a way I can pay for something, such as an in-app purchase, using my credit card while saving the iTunes balance for something else?
No You use the iTunes gift cert until it runs out. Then it will auto switch to your payment method.