Functional field value update

Hi Team,

I am using OpenERP 7.0 on Ubuntu 12.04. In purchase order amount_untaxed, amount_tax, amount_total fields are functional fields and the values are stored from purchase.order and purchase.order.line models only. I developed a new purchase amendment module, there user will change tax value and click on Amendment "Confirm" button i am updating the latest value in the purchase order. In this case if any changes in tax the new value not updating in purchase order. Kindly help me how to solve this and feel free ping me i u need more information about this.. Thanks in advance