Receiving and matching complementary Items

All,

I was wondering how the complementory items are received during the item receival. For example i have ordered 100 items of product 'A'. for every purchase of 10 product 'A' items, 2 product 'B' items are given as complementary (buy 10 get 2). so when i receive the items, i will receive 100 product 'A' items and 20 product 'B' items. During receiving these items, how does these extra quantities ( these complementory items) are matched in 2/3/4 way matching.

My PO is for 100 quantity, Invoice is for 100 quantity and for how many items i should have the receipt and against which quantity, these are matched.

Please educate me understanding this scenario.