Other Limitations and Fees
You may exchange an item only once during the initial 30-day period. An exchange does not extend the 30-day period.
Sprint reserves the right to deny service to prevent multiple returns under the Sprint Free Guarantee
If you cancel service within 30 days of activation, you are required to return your device and will no longer be eligible for any associated mail-in rebates.
If you do not return your device within 30 days of deactivation, you will be charged an Early Termination Fee of up to $200.
Replacement devices provided as part of an advanced exchange that are not returned within 10 days of receipt of the replacement equipment may be subject to an equipment non-return fee of up to $500.
For any devices or accessories purchased through a Buy One, Get One (BOGO) offer, all devices or accessories purchased must be returned/exchanged to receive a refund or comparable exchange. Each upgraded device exchanged may be subject to the $35 restocking fee, except where prohibited.
source:
Sprint 30-day Return and Exchange Policy