How to grant free months to an auto-renewal subscription (iOS)?
<p>I have an iOS app with different auto-renewal subscription possibilities, and I want to offer it's user the chance to earn a free month when they invite friends to the app.</p>

<p>I know how to handle the scenario where I'd give a free month to a user that's not already subscribed, but I'm wondering how to handle a user that's in the middle of a year-long paid subscription and should receive a free month..</p>