Flexi can make sure that a received invoice only enters the payment order once someone has approved — "signed" — it. As long as the signature is missing, the invoice will not be offered for inclusion in the payment order.
Enabling the check
Go to Company → Settings → Modules tab → Purchase tab and check the Require signature before issuing payment order option.
Signing the invoice
Open Purchase → Received Invoices.
Above the invoice list, click the arrow next to the Services button and select Sign for Payment.
The Payment Order Signature column will be checked for the given invoice in the list.
💡 Only users who have permission for this service can sign invoices — by combining this with user roles, you can separate payment approval from invoice entry.