Managing Inventory with offered products

Managing Inventory with offered products

Dear all, 

I have a problem with the correct way to manage my inventory. I use Zoho ONE and Inventory is linked to Zoho Books.

Let's say I bough 100 items of one specific product. I intend to send for free most of the inventory to customers, and sell the remaining few items. I cannot think of a way to manage this. At first, the inventory was handed based on bills & invoices. I though that the solution was to go in "Receive & shipment" mode but I have got a message saying that since Inventory and Books are synced, it will always be based on Bills & Invoice.

Also, is there a way to prepare a mass update? Because this gift is sent to a lot of people in my database.

Thank you,