@John Siri Thanks for posting in our Q&A.Since you do not have access to the customer's account, you might need to follow these general steps:
1.Factory Reset: Perform a factory reset on the returned PC to ensure all previous data and settings are wiped.
2.Remove devices from Intune and Entra ID (previously known as Azure AD): Remove the device from Intune and Entra ID. If you don't have the permission to access the customer's Intune portal and Entra ID portal, it is suggested contacting the administer of the customer to do this action.
3.Re-enroll for New Customer: Once the device is wiped and removed from the previous enrollment, it can be re-enrolled for the new customer using the standard enrollment process.
Hope it will help.
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