Hi,

You can create a groups for specific employees like Managers and Employee.

By Using following code you can control showing approve button(After assigned, groups to specific employees)

<button string="Approve" type="object" name="approve_sales_data" groups="module_name.group_name"/>

If you don't know how to create a groups, following link will help you to creating groups in odoo https://www.odoo.yenthevg.com/category/security/  Otherwise you can use a HR Module groups(You can set a groups in Settings -> Users -> Employees)

Thanks.