When the Customer is set up to Bill by Customer instead of by File, the AR Balance is reported in the Subtotal for the Customer as an information field. Because the File number is not transferred to Receivables, there is no way to break the AR Balance down by File and report it in the details,

For Customers that are billed by customer, the AR Balance is not added to the Total
Outstanding subtotal for the Customer or to the AR Balance Subtotal for Responsible staff member.

I will add a suggestion to the wishlist to include the AR balance in the subtotals.

I believe that setting cannot be changed on the customer when there are WIP transactions existing for that customer. To be able to change this on the customer, you have to clear out the WIP in Time&Billing - which may not be preferable.