My mobile (HTC) has Webroot SecureAnywhere Mobile Premier as the AV it expires in 2 days. Currently displaying & has been for 20 minutes a splash screen stating Completing purchase... Will it ever complete the purchase?
I'm not clear the back button is getting you off the screen you're stuck at, which is what the intent was. The device shouldn't lock itself just from hitting back. You could try turning off the device with the power button and restarting it.
The problem in this case seems to be with the Market/Google Play itself. If it was Webroot that was stuck, we could look at that in greater detail. However, a frozen purchase is going to be the purview of Google since the app that performs that purchase is theirs. If a device restart doesn't help, you may need to contact Google support on this issue.
Are you doing this on the Market/Google Play? It sounds like it's stuck. I'd suggest hitting the back button. If the purchase went through it will let you download the app. If not, it would still prompt to purchase it.