Isupplier invoicing tax

Hi I am new to this and apologize if i am in the wrong area. We are trying to implement isupplier invoicing in our 11i instance and we are having issues with the sales and use tax.  While testing we entered the tax incorrectly to see if the invouice would go on a hold, however it is not and since the tax line is being created the invouice doesnt calculate tax it only takes the invoice as is.  Thank you for your help any and all help is greatly appreciated.