Unit Deals is a piece of functionality added in version 4.314 of c9 that allows you to build up a unit sale over a period of time, as a purchase order.
The intent of deals is to simplify the process of managing unit sales, purchases and tradeins by making the process more intuitive and more forgiving. Prior process for selling a unit in c9 required keying in all data in one sitting. Unit sales can be complicated animals, and expecting a user to key in everything correctly on the spot is something of a task.