Replacing item on Sales order

Business case: We have certain items to be manufactured in a different manner for specific business units. So we want to replace the item on the sales order with a different item if the sales order belongs to certain business units.

Example: Item A can be sold as such in BU 1 and BU 2. But if the same item A is added to sales order in BU 3, this item needs to be replaced with item B.

Problem statement: Item substitute works only when the original item is not in stock. If there is a means to replace or substitute the item if the SO belongs to a certain BU