You can use our google wallet for play store APP purchases but you cannot use play Store credit in your google wallet. I just bought the Nexus 7/Chromecast combo in the Play Store and I was not able to use my Play Store Credit to buy it. You can see your Play Store Credit if you open Google Wallet but it is not a Pay option when you select Pay Type under Google Wallet when I was making the purchase.
That sucks if it is sold in the Google Play Store why can?t I use Play Store credit?
On a separate but similar note I ended up canceling the order so I could get the better deal form Office Depot yesterday of $50 off the Nexus 7. I already had my money tied into Google Wallet so I was very lucky that Office Depot accepts Google Wallet. They did not know this until I tried it but it worked great! Now I have $90 on my Google Wallet account though.