here is the terms from google for Nexus 7
" In order to receive this promotional balance for the tablet activation, you must currently have or create a Google Wallet account on your tablet, and you must also add a credit card or debit card to such account (or already have a credit card or debit card in such account) for use in future purchase and payment transactions with the Google Wallet, by September 30, 2012. This promotional balance is not available for Google Wallet accounts where you only store your bank account or Carrier Billing information as a payment method."
Google Play Promotional Balance Terms section C2