Using a gift certificate for an app without fully redeeming it
There are two different types of Apple gift cards. iTunes cards can only be redeemed on iTunes, and conversely, Apple gift cards can only be redeemed at an Apple Store, the online Apple Store, or 1-800-MY-APPLE. What you are asking is not possible since Apple gift cards cannot be redeemed for apps. (Or if you have an iTunes gift card, it cannot be redeemed for physical goods.)