You can either lookup the previous balance or use it over a relationship. The former will not update automatically if you edit prior entries, the latter will get progressively slower as the number of records grows.
In either case, you need a relationship to link to the "previous" record. I didn't quite get your description, but it should be something like:
Invoices::AccountID = Invoices 2::AccountID
Invoices::InvoiceID > Invoices 2::InvoiceID
with records from Invoices 2 sorted by InvoiceID, descending.
Thanks, comment. I used the first option -- looking up Invoices 2::Balance to Forward from the context of Invoices after relating my tables in the way you suggested and it works in my test solution (which is pretty bulky by now)!