Call the function for the field price_unit in the view xml

<xpath expr="//field[@name='price_unit']" position="attributes">
   <attribute name="on_change">check_margin(product_id,price_unit)</attribute>
</xpath>

and in the class, define the function to process it.

def check_margin(self, cr, uid, ids, product_id, unit_price, context=None):
  res = {}
  warning = {}
  sale_price = None
  if product_id:
     sale_price = self.pool.get('product.product').browse(cr, uid,product_id).list_price
  if unit_price is None:
     pass
  elif unit_price < sale_price:
     warning = {  
       'title': _("Warning"),  
       'message': _('Unit price given, is less than the sales price of the selected product. Please change (or contact your sales manager to change) the sales price of the selected product.'),  
               }
     res = {'value': {'price_unit':sale_price}}
  elif unit_price >= sale_price:
     res = {'value': {'price_unit':unit_price}}
     pass
  return {'value': res.get('value'), 'warning':warning}