If customer have positive credit balance in his account, this credit can be applied to his due invoice by staff member.

1. Proceed to Payments->Invoices
2. Select customer invoice from list

3. If customer have credit available, a field "Client Credit" will appear as shown below
   

4. Use "Add to invoice" to apply credit from client profile to invoice.
5. Enter amount of credit to be applied, by default HostBill will load maximum amount available from client credit that is not higher than invoice balance
6. Click on "Apply from credit"
   
7. Customer credit will now be decreased, credit change will be logged in client credit changes log.