My only question is, whether anyone of you has ever implemented functionality like this in PopCommerce.
Scenario: imagine you receive a proforma invoice (usually shortly after you submit a purchase order to your supplier), which you record into the system (probably as something similar to mantle.account.invoice.Invoice). It is a document, which you treat as a liability. But you do not put it into costs, because you still expect a (full/normal) invoice to come afterwards, but you pay it. Later on, when this full invoice comes, you record it into the system. And preferably, linkink these two (invoices) together, because you are not going to pay the full invoice, as it was already paid on behalf of the proforma invoice. It is like having a single accounting case in two related documents.
Is a functionality similar to this implemented in mantle-usl? I have checked the tests in mantle-usl and I have not found it.