Opencart's Store's Customer Credit payment module allows approved customers or guests (depends on settings) to buy using credit. Administrators cant set Credit Balance limit or Customers Group that will be able to use Store's Credit.
Also it does inform Customers about their current balance using custom text message that can be altered from Module's settings.
Set MAx Balance
Set Customer Groups that can use Credit Option
Enable/ DIsable Credit for Guests
Custom Notifications in Every installed Languages including Current Balance