If you only looked at the inventory summary report, you might wish to look at the Inventory Quantity report.  It shows quantity, what's on sales orders and what's on purchase orders.  This information is on the inventory item ledger as well.

Many accounting software packages (every one I've seen, that wasn't Sage 50 Canadian) will show an 'Available' quantity, which is calculated by taking on hand less open sales orders and 'layaways'.

There is a warning in Sage 50 that 'one or more... caused inventory to drop below the reorder point'.  I haven't tested to see whether that will warn on entering an invoice that takes available on hand below the order point as well.

Failing that, there is no way to do what you need in the current software's order entry / invoicing module.

If such a feature would be useful to you, there is a 'Contact Sage' item under the Help menu, where you can leave Feedback.

the inventory/services section shows what is electronically available to ship
double clicking on the item shows how many have been purchased, and not yet electronically received
and how many have been allocated to other customer, and not yet electrically shipped
assuming that the computer information matches the warehouse, you could ship the next customer 22 batteries

if the customer wanted 26, you could find out when the four on purchase orders will arrive, and
   ship 22 today and 4 more when they arrive, or
   see if the customer can wait x days and ship 26 batteries

it's a good idea to drill down, the information could help make a sale - especially if you think you have none, and there a purchase order due to be delivered today with enough quantity to ship

Thank you very much everybody for the quick answer. It's not quite what I expected but I know Sage as many limitation so i will live with it.

Thanks again !

What this gent is asking is sage does not have a "RESERVED STOCK function " for stock that has already been placed and used on an open order, effectively removing it from the physical count so you do not oversell the available quantity .

Have 6 in Stock 

use 4 on an open invoice

leaving you with 2

Sage still shows you have 6 not 2,  because the inv is not posted so someone else is going wrongly see they still have 6 left to sell .

that should read "DOES SAGE NOT HAVE"  the ability to reserve stock issued out on OPEN ORDERS from the remaining stock