class model_A(models.Model):

    _name='module.model_a'

    sale_id = fields.Many2one(string='sale', comodel_name='sale.order')
    initial_price_value =???

class model_B(models.Model):
    _inherit = 'sale.order'

    model_id = fields.One2many(string='model_b', comodel_name='module.model_a', inverse_name='sale_id')

    initial_price = fields.Monetary(string="Price Initial", store=True, readonly=True, compute='_product_amount',
                                    tracking=4)
    amount_untaxed = fields.Monetary(string="Untaxed Amount", store=True, readonly=True, compute='_product_amount',
                                     tracking=5)
    amount_tax = fields.Monetary(string="Taxes", store=True, readonly=True, compute='_product_amount')

    @api.depends('order_line.price_total')
    def _product_amount(self):
        for order in self:
            amount_untaxed = amount_tax = 0.0
            for line in order.order_line:
                amount_untaxed += line.price_subtotal
                amount_tax += line.price_tax
            order.update({
                'amount_untaxed': amount_untaxed,
                'amount_tax': amount_tax,
                'amount_total': amount_untaxed + amount_tax,
                'initial_price': amount_untaxed + amount_tax,
            })