Code :

<xpath expr="//field[@name='cover_page']" position="before">
<field name="is_manager" invisible="1"/>
<field name="bill_ids"
attrs="{'readonly':[('is_manager', '=', False)]}"/>
</xpath>

is_manager = fields.Boolean(
compute="_compute_is_manager",
)

def _compute_is_manager(self):
self.is_manager = self.env.user.has_group("sales_team.group_sale_manager")