iTunes Store - Use My Credit Card Not Store Credit For A Gifted Item
Quote from Apple's Support page about Gifting...


  "When you purchase an iTunes Gift, you must have a valid credit card
  on file; you can't purchase an iTunes Gift with store credit..."
  
  - http://support.apple.com/en-us/ht2736


So to simplify.. 

Apple charges the gifted item straight to your credit/debit card. NOT the credit on your account.