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Totally understand it won’t suit all circumstances – With no current plans to add individual locks per invoice, perhaps the Invoice Only Draft role ( https://central.xero.com/s/article/User-role-access-to-sales-in-Xero?userregion=true#Salesinvoicesuserrolecomparison ) may be appropriate for some users and reduce the # of users in the org with the ability to edit approved invoices?

Another way to keep an eye on who which users are making changes to approved invoices would be to check the History & notes ( https://central.xero.com/s/article/View-a-history-and-notes-summary-for-transactions-and-user-activity ).