+This method cannot handle already partly assigned bank transactions, i.e.
+a bank transaction that has a invoice_amount <> 0 but not the fully
+transaction amount (invoice_amount == amount).
+Currently this state is impossible from the point of the user interface,
+but for double safety and further reliance posting an bank_transaction
+where some invoice_amount is already assigned or a RecordLink from
+bank to document exists will not be accepted.
+
+If the amount of the bank transaction is higher than the sum of
+the assigned invoices (1 .. n) the last invoice will be overpayed.
+
+Therefore this function implements not all valid uses cases.
+