OpenERP can not save the readonly field in sale order

Salam ,

In a sale order line, I try to make price_unit readonly for certain group , so to do this :
1- I created 2 groups : Sales / Readonly price unit & Sales / Write price unit
2- I used fields_get to redfine rights access like this :
  if group.name == 'Sales / Readonly price unit':
                        res[field].setdefault('states', {})
                        res[field]['states']['draft'] = [('readonly', False)]

                    if group.name == 'Sales / Write price unit':
                        res[field].setdefault('states', {})
                        res[field]['states']['draft'] = [('readonly', False)]

Everything works fine except that by recording the sale order the unit price is set by default to 0.00 for users who have the readonly group.

Thanks for your help