By design of Unleashed's Xero integration, the Invoice Date provided in Unleashed's Sales Invoice will always be the date that exports to Xero when the invoice is completed in Unleashed.  Invoice is in Parked status.

For more details on the field mappings for Sales Invoices exported to Xero, see Xero after the integration setup: Field Mappings

Within Unleashed you can set either the Sales Order's Order Date or the Invoice's Created Date as the auto-populated Invoice Date in each Sales Invoice. But you will still have the opportunity to manually amend the Invoice Date when required, whilst the Sales Invoice is in Parked status.

To set the default Invoice Date, see the Company Settings: Invoicing tab. 

NOTE: If you require the Invoice and Order date to match in Unleashed, our recommendation would be to create the Order then Complete it's Sales Invoice on the same day. This process will however depend on your business workflow and may not be suitable for all cases.