I'm pretty sure google isnt just going to give you ten bucks because you got ten bucks on a prepaid card with your phone. from the sound of it you didnt buy any credit on the prepaid card. I did receive my refund for the money i had put on my prepaid card before they just offeredc any debit card a long time ago.
thats like if apple said "hey free $15 gift card with a new iphone 5!" "wait nah we're not gonna offer the itunes cards anymore" and gave you the 15$ instead.