Parent child Item Relationship Buying and Selling

We have business Use case where Client is buying in different Item Code and Selling in Different Item Code

Lets Say, we are buying from supplier as Box and it Contains 12 Units in each Box and We are selling in Units to Customers, Box has separate Item code (1234)and Each Unit has separate Item Code(7890), When we place Purchase Order, we do order in Box (1234)and also receiving into Same and Stock is up in 1234 Item but when we have Customer order, its in Separate Item Code (7890) but stock is available in 1234 Item Code as we have done receiving in 1234 Item