Show total outstanding amount of client account


I am using the outstanding amount field in my sales invoice in order to show of course exactly what it says.

What I realised is it shows the outstanding amound of the current invoice that I am submitting and not of the whole client account.

For example if client has a debt of 20€ and I submit another invoice of 15€ then the last printed invoice will show 15€ on the outstanding amount and not 35€

How can I fix that? Or is there some other field I should choose for that?