It is not possible to sell products with a negative stock balance. If you need to issue a sales invoice to a customer but the products are not yet available:

Reference book → Item Cards → Items → Click the green plus + button → Create item cards with the attribute “Services” → Save.
In the module "Warehouse" → "Sales," use the same item card, but adjust the name in the "Description" field.

There are two possible ways to adjust the warehouse after selling goods marked as a Service:

a) If the goods are received into the warehouse with a date earlier than the sale, you can replace the items sold as Services with the actual goods that were not in stock at the time of sale.
b) If the goods are received into the warehouse with a date later than the sale, you will need to create a write-off act in the module Warehouse → Sale.