Hi Rony2
Thanks for your post and Welcome to MYOB Community Forum.
If an unpaid invoice is showing paid already, a payment transaction was applied to that invoice to close it. In that case, you need to reopen that invoice then you would be looking at deleting that customer payment which would make that invoice open again Here's a Help Article on Deleting customer payments that you can follow.
Please let me know if you need further assistance.
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Best regards,
Doreen