When it comes to official business documentation (USA) between a company and a client who is either another company (B2B) or individual person (B2C), how can the company providing the invoices, prove that those invoices are really not yet paid?
Passed due date is obviously insufficient proof. Also payer's (client's) bank account statement, with missing relevant line(s), is insufficient as well because the payer could have paid with a different bank account.
What else could be done here? Is there any official definition of such USA document and where could I get it?