The suggested link provided by Niyas,  solves the problem by setting a conversion rate (equation) between the base UoM and the selling UoM. ( Purchasing in Gallons, and selling in Liters) as both has the same Category (weight). This is done on the product card. So you need to define the boxes within the same family. For example call the purchasing unit Box15, and the base unit KG. which means you will end creating Box5,Box7, Box19,...etc as a new unit of measure in the same category. 
In other software ( like ERPNext and others)  they solved it by allowing the user to add the conversion rate  at the level of the item line on the Sales/purchase order.  This issue is important for businesses working in food industry like fruits and vegetables. May be we can work on a module to solve this issue. 

This can not work from a logical point of view. How do you want to track stock? You'll have to track your Mangos in kg (and/or pieces maybe, if at all) and boxes can only be packages with no UoM functionality (each box has a different weight). So you can only buy your mangos in kg, store them in kg and sell them in kg, no matter if they are packaged in boxes or not.