System Migration: Existing Balances - Credit Balances

In preparation for switching to Dolibarr as a billing platform I am importing customers and their balances. I am creating an Invoice to reflect monies owed, but cannot identify a way to reflect credit balances. (e.g. An account has a $10 credit balance and all their invoices are paid.

How do I initialize a customer in Dolibarr with a credit balance that will automatically be applied to their next invoice.