How to record advance payment received from customers in previous year as opening balance for new year?

How to record advance payment received from customers in previous year as opening balance for new year?

I am able to create debit balances for customers as opening balance, but how do i create a credit balance for a customer?
Some customers have paid me in a previous year and their opening balance will be credit and not debit. When i try to import csv with a negative number, there is an error?
In the openeing balance section when I try to import csv for Unused Accounts or other accounts, there is an error that these customers cannot be linked to that account !

Please advice