Project Billing -Credit Memo Sales tax Issue

Hi, I am having the issue with  Project Billing Credit Memo. Here are the steps I am following:

1) Entering TimeCards using Oracle Internet Time.

2) Save and approve time card.

3) Run "Transaction Import"

4) Distribute Labor Cost

5) Generate Draft Revenue for a Single Project

6) Generate Draft Invoice for a Single Project

Approve and Relase the invoice. Print invoice. This invoice is fine, it has correct sales tax on it. Everything is good.

I go back to the same Timecard in Oracle Internet Time. I update the timecard and reduce the timecard hours from 8 to 4.

Run the same steps again. This time the Credit Memo gets generated to adjust the amount.