Dear all,
I need to clarify the issue more. The process has two main steps:
1. In actual we buy a material which consist of several thousands of materials, but in SAP we define it as a material number without BOM and buy it as a whole.
2. Sometimes in actual we have to use one of these components for in-house consumption. (we don't have enough time to buy it separately and also we have no idea which component may be needed in the future). Again in actual we will buy this component and will substitute it after a while, but we do nothing in SAP for this step.
Sorry about confusion.