how can i add in the sales invoice the option, previous balance and new balance for a client that can be automatically updated every time a new invoice is printed?
You cannot. That is not what an invoice does. What you want is a customer statement, which you create under Reports.
To include recent charges, you must create a new sales invoice before creating the customer statement.
By recent charges you only mean as of right now/today, yeah? Wouldn’t the report be current of all sales and payments to date without the need of a new entry?
In writing this I think I’ve come to understand what you mean. In the OP’s original scenario, you would need to create the new invoice/sale and THEN print the customer statement.
You are correct on both points.