Hello Medmars,

In py

class AccountInvoiceLine(models.Model):

     _inherit = 'account.invoice.line'

     unity = fields.Char('Unity')

In Xml

<record id="account_invoice_line_tree_view_inherit" model="ir.ui.view">

    <field name="name">account.invoice.line.view.inherit</field>

    <field name="model">account.invoice</field>

    <field name="inherit_id" ref="account.invoice_form"/>

    <field name="arch" type="xml">

        <xpath expr="/form/sheet/notebook/page[1]/field[@name='invoice_line_ids']/tree/field[@name='quantity']" position="after">

            <field name="unity"/>

        </xpath>

    </field>

</record>

Hope it will works for you.

Thanks,