Based on a current promotion with Real Racing 3 that's offering free in-app purchases, the ability for app developers to hand out promo codes for in-app purchases may be in the works. Currently, those who purchase Real Racing 3 (an EA title) are being offered a "Handful of Gold" via an email promo code to receive in-game currency, which is then redeemed in the App Store.
Full story from the iMore Blog...