Unit mapping is the concept of having more than one unit per item and also we can price that item based on the units. Both sales and purchases prices will get change based on the unit mapped to the item. The Real inventory is always with us in Primary unit. The Primary is the unit that you are giving while creating item/ service. Which is also the minimal quantity unit as we need to choose it.  Let's go with an example, If we take pencil as example. 
Piece is the primary unit. Box, Dozen, Carton, etc are the secondary units, which are going to be mapped with item and which will be added the quantity based on the primary unit Piece.

Demo Credentials

Username : testerp
Password. : Test@123

Here is the working example. 
1 Piece     = 1 
1 Box.      = 10 Piece
1 Dozen.   = 12 Piece
1 Carton.   = 100 Piece

Unit Mapping 

This is just an example illustration. It won't be same in the real time scenario.  We have to feed the secondary units against the Primary unit.  And the system will help you to the prices based on the Mapped unit. 

Sales Price

It can be priced in both sales and purchase prices.  Let's go with Sales price along with units and Sales type. We have the sales types which is already there with FA. And we are connecting it with Unit mapping. 

Purchase Price

And the Purchase Price will be like this. 

Purchase Module 

We can now use the drop down in the units column of Sales and Purchase forms. Which will help you to change the units for the selected item. And also You can get its price as per you defined here. Let's check with Purchase Form.

Sales Form 

And here is our sales form with units drop down. 



And the real transaction will be done based the quantity of the units assigned against base unit. And you can see the reports with normally the base unit.  Which will help you to know the real stock with base(Primary) Unit.

Note: This is a core customized program, not an extension to install it easily.  You need to customize your version of FA with it. And If you have php programming knowledge its good to go, or if you have developer with you, it's great to work on it. But the non-technical people, kindly use a developer to do this. If you want us to do this merging to take effect in your version. it will be additionally charged with it. it doesn't cover in the module cost.