• Navigate to Settings > Selling Channels.
• Locate the Amazon store you’d like to change and click on the ellipses icon. Then click “Edit”. 
• On the popup page, locate the field labeled “Deducting inventory from FBA shipments.”
  
• Click on the dropdown menu to select your preferred option for deducting inventory. You can choose between deducting inventory when the shipment is in “Working” status or when the shipment is in “Shipped” status. Select the appropriate option based on your inventory management preference.

Note that on the item activities, the FBA shipment will always be displayed on the date of shipment creation. The above-mentioned setting is only applied , when the inventory should actually be deducted.