from openerp.exceptions import ValidationError

#below code is your if condition
if price is less than:
    raise ValidationError("Price is below") 

Hi Yustinus, 

If you need to show a warning in pos interface when user input a price lower than the standard price, you can achieve this by modifying the function "set_unit_price". This function is used to set the unit price for the products in the orderlines. In your case, you can add the warning based on your condition inside that function. 


Hope this helps.