There are multiple ways to handle putting items that are not in stock on an invoice, use the method that suits your business the best.
Option 1: Restrict sales to available inventory items only
If you turn on the option in application settings to restrict sales to available inventory items only, TSS will not allow you to post the invoice without an available item. You can special order the item from the functions menu which will automatically place it on a purchase order, but the invoice will not post until it has been received into the system.
Option 2: Allow sales of items without available quantities
You can allow the sales of items with no available quantities but this requires your inventory manager to reconcile these quantities when he receives in purchase orders.
The quantities that have been sold in the system that were not available at the time of sale can be reconciled with the print physical inventory report.
Set your report criteria to a transaction type of 'MANUAL', a transaction date range in the recent past, and the make or model to the product(s) you are going to receive on the purchase order. When you run the report it will show you a list of any items that were added to the system in that date span outside of a standard purchase order. If it was received to accommodate an invoice it will reference it in the invoice number column. This will allow you to not re-receive in items a second time due to them being sold before the purchase order was received into the system.