Cannot purchase Infuse Pro from within Infuse 2 app

I purchased the Infuse 2 app awhile ago but removed it from my iPod Touch 5 because I ran out of space on my iPod.  However, I just redownloaded it because I will be getting a Seagate Wireless Plus external hardrive on which I will store all my movies for streaming.  After reinstalling the Infuse 2 app, I am offered an in-app purchase for Infuse Pro for $6.99.  Everytime I try to purchase Infuse Pro, it asks me to enter my iTunes password but gives me an error saying I have either entered the wrong iTunes username or password.  I know for a fact I am entering the correct iTunes username and password because the same credentials work properly for everything else on my iPod. 

Why does the Infuse Pro in-app purchase keep insisting that I have entered the wrong password?

This is a quirk we're working to resolve for the next update.

In the meantime, you can try restarting your iPod (this will cause Infuse to refetch the purchase receipt upon opening Infuse) then tapping the Restore button should allow you to redownload Infuse Pro without having to pay again.