revenue event in Project billing

Hi, I am totally new in Project and I have an issue that I can not resolve.

I am in r 12.1.3

We are using Project billing.

I have created a revenue event for a project in march 2015 (in r11, later on in agous we migrated to 12.1.3). The revenue event was for 30 euros. The project has 2 agreements.

Now I discovered that that revenue event had been assigned to the incorrect agreement.

So today (december 2015)  I created a negative revenue event, to the same agreement for the same amount but negative (-30 euros). When I run the PRC Generate Draft Revenue for a single project, the process generate the event but with amount 0. No error message, nothing, only amount 0.