Best Buy is the latest retailer to start offering customers a trade-in option to get a new iPhone, but its offer only stands for a week.

The retailer will offer customers a $100 credit they can use toward purchasing a new iPhone 5C or iPhone 5S smartphone, provide they trade in "any working smartphone."

The iPhone 5S is a bit more expensive, but that $100 credit will allow interested buyers to get an iPhone 5C for no cost if they trade in their old smartphone at Best Buy.

The news comes from Techno Buffalo, which managed to get its hands on an internal Best Buy document (see above) detailing the upcoming promotion.

"The offer runs from Oct. 13 to Oct. 19 and is good for customers who trade in any working smartphone," Best Buy told Mashable in a statement. "If a customer's smartphone is more than the guaranteed credit, they will receive the positive difference in the form of a Best Buy gift card."

At the same time, if a customer trades in their old smartphone toward the purchase of a new iPhone, but that iPhone model is not available in store at the time, the customer will receive a "raincheck" offer. Best Buy also noted that additional costs such as carrier activation fees or sales tax may apply, so be careful when making your purchase.

While the offer will typically start on Oct. 13, participating Best Buy Mobile stores have kicked off the deal early on Thursday, Oct. 10, for the lower-cost iPhone 5C smartphones.

Apple has also launched its own trade-in program in late August, allowing customers to put their older iPhone models toward the purchase of a new one. To qualify for this deal, customers had to trade in their existing devices, receiving credit based on the model and condition of the smartphone they traded in.

Following Apple's trade-in program, several other companies, including Target and Walmart, have kicked off their own trade-in programs for customers who want to purchase a new iPhone. Best Buy is now joining the party, but it remains unclear at this point why its trade-in offer will only be available for a week.