Create admin account that only has access to select user accounts - Microsoft 365

you can create a restricted admin account in Microsoft 365 by setting up a custom role group in the Exchange Admin Center (EAC). Assign roles like Mailbox Import Export and limit the admin’s access using a custom recipient scope that only targets the users being migrated. This way, the buying company's IT person can manage only the selected mailboxes.

https://learn.microsoft.com/en-us/exchange/permissions-exo/permissions-exo