Hi there,
Digging up this old post.
I have a similar problem with Google Play credits.
I have credits from the Google opinion rewards app. Also my phone carrier supports carrier billing and I have a reasonable amount of unused credits there usually. The problem is all these credits are bounded as AUD (Australian dollars). There are a couple of apps and games that supports IAP but they only accept USD or JPY. When you try to pay using the carrier billing option, you will get a message "Purchase currency is not supported". I went as far as asking my carrier and they say it's not something that can support "in the near future".
There was also a closed thread about similar request:
http://forum.xda-developers.com/showthread.php?t=2103287
Someone could possibly write an app that supports IAP, and then transfer that amount to anywhere in other means (e.g. PayPal). Of course you will lose part of the credits since Google charges 30% for IAP, and also PayPal has a fee (I can't remember exactly what and how much), but still better than nothing (depending on how desperate you are I guess).
Then of course there's also the problem of trust here, and how to do this securely.
Any thoughts anyone?