iPhone 5 serial number changed after repair??

Brought in my iPhone 5 to have the screen and battery replaced before selling it. The buyer reported it is asking for user ID as it was locked. This was strange as i had already restored it and made sure it was unlocked. <br> I contacted Apple to help me unlock it, only to find out that \somehow"the SN number does not correspond with the phones original SN. (?) brought it back to shop to ask what happened? <br> Their solution was - \""we will take out the battery and screen and refund you\"" - really ?!!! <br> the phone was working with the correct SN on it when i brought it and it has not been anywhere else but with myself until i brought it in for repairs. no i have a dud phone that i can do absolutely nothing with. <br> i have sent emails everyday after i brought it back to the shop"