I am trying to implement a Time Bank... any ideas on how?

I have a site where I want to sell people a block of time for service. I can do this with an e-commerce solution but I need to show each user how much time they have left in their 'time bank'. What is the best way to show them a value like this (not editable by the user). I don't mind the upkeep manually since the work we do is not in the Drupal site itself so we (admins) would need to log in an update their time bank after they used some.

I just need a way to show what they started with, when they used some, how much they used and how much is left. An option to top up the time bank would be good to have there too.

Anyone doing anything like this?

Thanks!