@Saitam

Try this, it works! I created 'commitment_date' field in 'sale.order' and added it to 'account.invoice':

class AddCommitmentDate(models.Model):
    _inherit = 'sale.order'
    commitment_date = fields.Date('Commitment Date')

class AddToAccountInvoice(models.Model):
    _inherit = 'account.invoice'
    sale_order_id = fields.Many2one('sale.order', 'Orders')    
    commitment_date = fields.Date(related='sale_order_id.commitment_date')

<record id="test_commitment_field_id" model="ir.ui.view">        
    <field name="name">Add commitment field</field>        
    <field name="model">account.invoice</field>        
    <field name="inherit_id" ref="account.invoice_form"/>        
    <field name="arch" type="xml">            
        <field name="date_invoice" position="after">                
            <field name="commitment_date"/>            
        </field>        
    </field>        
</record>

@ Art

It doesn't work like this. There is no connection between sale order and account invoice. If I add the following line to account_invoice.py I get a 500 Internal Server Error:
`commitment_date = fields.Date(related='sale_order.commitment_date')`

I am writing this as an answer and not as a comment, because I am new to this forum and I only can comment with 8 carma points but have only gotten 7 carma points with the registration here (which makes absolutely no sense!) .

This my model:

class AccountInvoice(models.Model):
    _name = "account.invoice"
    _inherit = ['mail.thread']        
    _description = "Invoice"        
    _order = "date_invoice desc, number desc, id desc"         
    
    commitment_date = fields.Date(related='sale_order.commitment_date')     
    
    ...

And here is my view code, but the Odoo server doesn't start after the modificated model, so I don't even know if the view code would work or not:

<div name="commitment_date" class="col-xs-3">                    
<strong>Date:</strong>                    
<p t-field="o.sale_order.commitment_date"/>                
</div>