First of all, the store should be able to accept returns without the manager being present. If they have a return policy in writing, anybody should be able to process it. Sounds kind of fishy to me in the first place. However, if you're unable to get the store to budge on returning the item w/o the manager being present, you still have options:
I would send the manager a letter or email (so you have it in writing) that you'd like to return your phone before the 30-day period ends. Then call Sprint and cancel the contract. You should then be able to purchase the phone at a Sprint store or another 3rd party retailer of your choice. Just make sure you return the phone ASAP when the manager is in to avoid a big charge on your credit card (assuming that's how you made the purchase) for the full retail price of the phone.
If you'd rather buy it from the original store, just cancel the contract with Sprint now and return the phone ASAP when the manager returns. Most places give you a few days to return the phone after cancelling a contract before they charge your card the full retail rate.
Whatever you do, make sure you put your intent to return the phone IN WRITING before your return period is up. Sprint may not accept the phone return, but they should be able to process any changes/cancellations with the contract.