@Ermin: In a credit workflow connected to the invoice the Total amount has to change becose this is on the invoice you gonna send to the customer. The Amount Due is not on the invoice (this is just a internal information) and that is fine like it is.

@Ray: yes I agree with you, this is working fine but the concept is wrong. The credit has to change the Total amount as well.

The workflow is like:

1) you create new invoice about 100 bugs you gonna send to the customer X

2) odoo is showing the credit 30 for this customer, you reedem this credit for this invoice

3) you create PDF for this invoice and like to send this invoice to customer -- but you see on invoice that Total is still 100, but you like to send invoice about 70 becose the credit for 30 was used for this invoice.

>> workflow is broken. Its not possible to create invoice with the correct amount of 70

I just tried this and it worked fine.

After creating the first Invoice and overpaying it, this is what is shown at the bottom of the second Invoice BEFORE applying the credit:

This is what is shown AFTER applying the credit:

This is what is shown to the Customer when they are emailed the second Invoice:

Perhaps you have something misconfigured?  Check how this works on http://runbot.odoo.com to verify on a correctly configured system.