Unable to setup AP Invoice approval for the condition 'Invoice line date less than PO creation date'

Summary: Unable to setup AP Invoice approval for the condition 'Invoice line date less than PO creation date' as the attribute PO creation date is not available in Approval workflow spread sheet.

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We are trying to setup approval for such invoices where Invoice lines date is less than PO creation date. But the attribute PO creation date is not available in Approval workflow spread sheet. Please let us know how we can setup this approval with or without adding the PO creation date attribute.

Version (include the version you are using, if applicable):