Requirement on Item within an Item

Hi,

We have a requirement where Say Item A is the main item but an item B within A forms the major cost(~80%) of overall Item A and hence business wants to track the item B separately. Also customers can bring in only B and receive a credit for the same, Also when the item A ships, B should be hitting Deferred COGS separately. What is the broad level solution to achieve the same?

Regards

Sandip