Users Allowed Partial Payment with Existing Google Play Balance

Multiple users (in the US and the UK) are seeing the ability to pay for applications (and in-app purchases) by applying partial payment with their remaining Google Play balance. This is indicated by a check box within the buy dialog box that you can check or uncheck before approving payment. This looks to be a slow and server-side rollout though since not everyone has this option.

source: xdadevelopers