You can inherit the sales order lines model in the sales module and make a method with any of Odoo api decorations for example. @api.constrains or @api.onchange to notify the user with a ValidationError if he is not allowed to change the price

Example:

class SalesOrderLines(models.model):
    _inherit = 'sale.order.line'

    @api.constrains('unit_price')
    def _const_price(self):
        #your condition and raise ValidationError

Happy to help :) an upvote will be awesome