AT&T trade offer for 100 dollars?

I have a iPhone 4 but it isn't mine but I have been using it for months now and I just found out about the offer to get 100 dollars off my next smartphone. When will this offer end cuz my contract ends in June. How can I make this work? What are the requirements of getting and trading for new smartphone?

Don't do it! If its in good cond. It can go up to $200 on craigslist and you get your smart phone for the price and pay ahead on your bill hope I help your choice though if you want to get low doller its fine so yoy don't have to deak with it