You can return the phone, but not the airtime.
You can either
(1) Return the phone and keep the airtime. Then find a store that has the phone you want, buy another $100 in airtime and get the 2760 at half price.
(2) Keep the phone. Find a store that has the phone you want, but another $100 in airtime and get the 2760 at half price. Then try and sell the unused SIM and phone on this forum (you should be able to get more than $30 for it, maybe up to $50??).
Either way, you're stuck with the $100 in airtime you bought. Having $200 in airtime on your phone isn't bad, since it does last 365 days. And a year from now if you still have a lot of airtime left you can buy a $25 airtime card and extend the expiry date for another 365 days. And if you still think that the extra $100 in airtime is still too much, then you can always try and sell it on the forum (although you'll probably end up with a little less than $100 for it).