After an invoice (sales or purchase) has been exported to accounts, Match-IT does not allow that invoice to be unapproved.
If you go into the invoice detail and select the Audit tab, you can clear the Exported On date; after which Match-IT will allow you to unapprove the invoice.
i.e. You are manually taking responsibility for the potential inconsistency of an invoice in accounts and now potentially removable in Match-IT.
We protect the unapprove action like this to conform to BS/ISO requirements.