In purchase order line i need to show the list_price of the product but that have a condition, today(may 30) if i make a purchase then the list_price is 100 will shown and after confirming the purchase if i change the list_price to 120 in the purchase it still needs to show 100 for that i have create a function with api.onchange()
It works well, 
but if i create a replenish with buy it creates a purchase order in that order the list_price is not updating
below is my onchange code

class PurchaseOrderInherit(models.Model):
    _inherit = 'purchase.order.line'
    
    mrp = fields.Float("MRP", help="Load from Product MRP(Latest)", digits='Product Price')

    @api.onchange('product_template_id', 'product_id')
    def _compute_mrp(self):
        for rec in self:
            if rec.product_id:
                rec.mrp = rec.product_id.list_price
            else:
                rec.mrp = False

Kindly help me