You will need to enable the Sell and purchase products in different units of measure feature to do this.

• Go to Inventory > Settings and enable the Units of measure feature in the Products section. 

• Go to Inventory > Configuration > UoM (under Units of Measures) and configure a new UoM for Box in the "Unit" category and specify the appropriate conversion between Units and Box.

• Go to the Product template records of the product(s) and specify the base UoM as Units. You can select either Units or Box for the Purchase UoM. 

The Inventory accounting will be done in Units and you will have the option to select either Units or Box as the UoM in the Sales Order. The conversion between the UoMs will be automatically handled by the system. You will also be able to process the return in Units since the Inventory accounting will be in Units.