Pastel xpress V18 - Service Items SHOWING Negative

[Globaltech]

Good Day Ladies & Gentlemen

I only started using pastel Xpress last year, I am still learning how to operate it effectively.

I run a Computer retail & Services business - The Services we provide are Computer repairs,Network Installations,CCTV Installations,Access Control Installations.

My question is i input the service items as services in inventory, when i process a tax invoice the service items are showing negatives In inventory yet its a service item.Why is that happening.

This is how i process a tax Invoice for a service:

item code:
Description : Computer repairs
Quantity : 3
tax:01
Exc Price: 300
Net: 300

AS soon as i update the batch for the invoice it decreases the quantity in inventory....

Why is it doing that when its a service item???

Can anybody help???