Overwrite Tax on PO

Hello,

How to set up tax Exceptions for Non Catalog Items as such Items are not defined in the Item master.So, there is no way for us to set up exceptions at the item level or using the Product Fiscal Classification

We have setup a rule under Determine Tax Rate in order to be able to select the Tax Classification Code with the tax rate we need to apply, We are able to do that, but when we apply it the tax does not change.

Please help.

M.