Costs on PSP (project) not invoiced further ECC 6.0

Hello,

I have the following issue on projects (WBS) with example:

1. 100 USD posted via activity on WBS element                                         100,--               o.K. on WBS

2. 200 USD posted via external purchase invoice on WBS element               200,--               o.K. on WBS

                                                                                                 Total       300,--

Resource-related billing:                                                                            100,--               not o.K. as 200,-- USD are missing.

Result: It is (at present) not possible to do a resource-related billing based on costs which are not entered via activities.

Is it mandatory to have "materials" for every cost element affected?

Perhaps some has an idea.

Thanks a lot in advance

Thomas