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Sprint wants your iPhones, will pay $100 even for old models

Sprint iPhone
Sprint / Apple

Thinking of switching carriers, and have an old iPhone laying around the place? If you send it in to Sprint, they'll give you at least a hundred bucks towards the purchase of a new iPhone 4S. You'll have to sign a new contract, but if you were looking to switch and sell anyway, this promotion could be a good chance to put a few extra clams in your pocket.

Sprint already has a buy-back program, but old iPhones like the original and 3G might only fetch you $40 or $50. Starting Saturday, they'll make up the difference to $100, as long as you're looking to pick up a 4S.

Of course, you could also go on Craigslist and sell the same phone for far more directly to some local buyer, but if you're wary of being scammed or don't want to bother with the process, Sprint will be happy to have you.

Spring says the promotion is for a limited time, but the fine print says you can trade in as late as August, so there's no big hurry if you have a month or two left on your contract. Here are Sprint's instructions in full:

1.  Go to sprint.com/iPhone to buy your iPhone 4S through our online store and open a new line of service (excludes upgrades)  2.  After you receive your new iPhone, visit sprint.com/buyback and click Trade in my device to receive a shipping label via email. (Note: If your Buyback value is less than $100, we'll make up the difference with an additional account credit. You'll need to activate your new phone by 7/3/12 and trade-in by 8/14/12.)  3.  Receive a credit on your bill within 2 to 3 billing cycles. Your credit may be spilt into 2 payments. Please wait the full 3 month billing cycle to receive your full $100 credit.

In other words: buy new iPhone, send in old iPhone, get money. Hop to it.

Devin Coldewey is a contributing writer for msnbc.com. His personal website is coldewey.cc.