how to view unapplied balance of payment on open items view of customer account

When i drill down into a customer payment, i am presented with a payment screen- it shows on the right hand side a blue box called summary- it has the applied and unapplied amounts.. eg on a payment of £700.00, £600 was applied to an invoice and it leaves an unapplied amount of £100.

In the customer screen that shows the customers invoices, and payments under “billing status open” it will show a line for the full £700.00… i’d like to see a column that shows how much of that £700 is left unapplied (without having to drill down on the payment line) Have tried to customise view but I cant find an appropriate field for unapplied balance

Any ideas?

BG