In openERP, once a purchase order is confirmed we can not modify it anymore. Purchase amendment module enables the provision to make some changes like increase or decrease in product quantity,unit price etc in a confirmed Purchase order. But we can edit tne order lines only when no payments are done and the products are not fully recieved.
Lets see how this module works,
Here i am creating a new purchase order with 2 products, and i have confirmed it,