Manage customer credit on account

[Eutanasio]

Hi,

 

Sometimes my clients want to keep credit on their accounts for their next orders. I haven't found a module for this, so I'm just using the private notes on the BO.
If there isn't a module, how to create a field like so I can edit the credit from the BO and the customer to see it from it's account on my-account.tpl?

 

thanks!