I tried to reproduce your issue in an IDES sandbox, but I was not successful.
Do you have some scales, or some additional formula in the pricing procedure?
Or some user exit causing problems?
No idea...
I will share my settings, so that you can compare them:
I believe it is not the pricing type, that is causing problems (unless there is a user exit implemented which overrides the standard behavior).
It is normal to get in the pricing analysis screen "Condition has been found (without condition record)" if you have never maintained conditions in VK11, but the manual condition should get copied from the sales order with pricing type G.
If you have done exactly the same as me and still face a problem with a fixed value condition determined with zero value in the billing, try to perform analysis with transaction PRCAT - Analysis Tools Pricing. It is very well documented in SCN - just perform a search in the forum.
If you have difficulties understanding parts of the source code in a formula/user exit, I would suggest to ask a colleague from the development department to help you in the investigation.