Disassemble a purchased item

We have to purchase an item from a vendor.  We need to issue PO for this top level item and receive it on the PO.  This top level item is serialized item.

This purchased item is a assembled product and has 5 different items inside that box.

After we received the item we need to disassemble it and use all of the sub 5 items inside a discrete job in order to assemble a finished good.

3 out of those 5 parts are serialized.  We are using average costing.

We created a non-standard discrete job and wip issue the top level purchased item to the job and did a wip return of the 5 parts from the job.  The top level item  is issued at $100, but the wip return happened at $0 for all those 5 parts.  Hence caused the material variance.  This is causing the average of those individual items to be wrong.  How to do the wip return at the pre-defined cost so that the material variance will not caused.